Installation - Wikibooks, open books for an open world

[PLEASE STICKY] How to avoid wasting your time and everyone else's time, and increase your chance of solving modded Minecraft crashes.

First of all: Don't post crashes on this thread. Create a separate text post. Obviously.

Summary

Know the common crashes

java.lang.OutOfMemoryError: Close your /FeedTheBeastCrashes browser tab, give yourself a little self-facepalm, and allocate more RAM (some packs have RAM recommendations, look for that). Problem solved. If not, go to a bug tracker if you suspect a memory leak. Make sure to isolate the problem.
# There is insufficient memory for the Java Runtime Environment to continue. # Native memory allocation (malloc) failed to allocate 1577296 bytes for Chunk::new 
Paradoxically, this can mean you've allocated too much RAM, causing the java heap to exceed the physical amount of RAM available. Try lowering the maximum RAM allocation. Also try closing web browsers and other large software suites.
net.minecraftforge.fml.common.MissingModsException: or java.lang.NoClassDefFoundError: Usually, this means you forgot to install a dependency. Look for hints in the crash report. The curseforge websites has a button to show all dependencies of a given mod, check that. If you still can't find a compatible dependency version, post here.
java.lang.RuntimeException: Invalid id 4096 - maximum id range exceeded. You've added too many mods and have run up against Minecraft's item ID limit. Either remove some mods, or use JustEnoughIDs (1.12) or NotEnoughIDs (1.7.10 - 1.12.2). I'm not sure what we did before 1.7.10 to solve this, but a solution exists. If anyone remembers what it was, post it, because my googling is failing.
java.lang.ClassCastException: class jdk.internal.loader.ClassLoaders$AppClassLoader cannot be cast to class java.net.URLClassLoader (jdk.internal.loader.ClassLoaders$AppClassLoader and java.net.URLClassLoader are in module java.base of loader 'bootstrap') You need Java 8. Minecraft does not support newer Java versions (9, 10, 11)

Crash reports & Logs

If your /FeedTheBeastCrashes post or bug report doesn't have any computer-generated text, then all we can do is guess. This is the most important part of this guide. Please also read the section below about pastebins
  • Post crash reports. Crash reports begin with ---- Minecraft Crash Report ---- and end at the end of the log file. Yes, the whole thing. Not some random line that you chose because you think it might be the problem.
  • If there is no crash report you can find, or you suspect it might provide additional context to a crash report, post the entire log file. If the problem is not a complete crash, it might be a good idea to provide the timestamp that the problem occurs at. It really doesn't matter if you think that the logs won't give any information. Post them anyways. If the Minecraft window opened at all, then it produced some logs.
  • How to find logs:
    • MultiMC https://github.com/MultiMC/MultiMC5/wiki/Minecraft-Logs
      • If you are using MultiMC and you are not posting a ---- Minecraft Crash Report ----, then along with the rest of the log, include the magenta text at the top of the log output. This includes useful information about hardware, the mod list, java version, java arguments, etc. Normally this is found in crash reports but MultiMC conveniently adds it to the top of the log output too. Hardware info is also available in all forge logs under [FML]: -- System Details --
    • FTB Legacy & Twitch https://ftb.gamepedia.com/Troubleshooting#Locating_Logs
    • Minecraft: %appdata%/.minecraft/logs/latest.log or ~/.minecraft/logs/latest.log
      • If you're using the vanilla launcher, do not copy from the log window, as this causes obnoxious formatting issues. Always find the actual log file.
    • Technic: %appdata%/.technic or ~/.technic
  • Indicate when the crash happens. As the game is loading? As your world is loading? The instant you place a certain block in the world?
  • If you're on a server, post both server and client logs. Communicate with your admin and try to work out what's happening.
    • Indicate the timezone difference between the server and client logs in your post, so we can match up what's happening on the server and client.

Pastebins - for posting logs

Avoid google docs. It loads extremely slowly, uses an annoying proportional font that breaks up vertically aligned plaintext tables, and makes it far too easy to screw up sharing settings to accidentally require a google login.
Avoid file upload websites such as Mediafire and GDrive. The log is not immediately visible and less people will go through the effort to download it.
Use a pastebin service instead, such as https://pastebin.ubuntu.com/ https://paste.feed-the-beast.com/ https://gist.github.com/. All of these except Github Gist can be used without an account. Pastebin.com is not listed because it has incredibly intrusive ads.
To use a pastebin, open the logs in a text editor like notepad, copy the contents with Ctrl+a Ctrl+c, and paste into the website text area. Do not set an expiration, as this causes link rot and makes it difficult for people to Google the same problem in the future.
If you insist on not using a pastebin, format your post properly by putting four spaces before every single line of crash reports and logs. Reddit Enhancement Suite has a button that will do this for you. This is perfectly fine, as long as you get the formatting right... just beware of the Reddit character limit.

Troubleshooting

Provide as much troubleshooting information as possible:
  • Google the error message, smartly. The error message is the line after the timestamp and description in a crash report, as well as anything after Caused by:. Then, when posting here, list out your troubleshooting steps and solution attempts in chronological order.
    • If this fails to provide useful info, include "minecraft", "forge", or the name of a mod with quotes in the google search.
    • If this still fails to provide useful info, try removing anything that looks overly specific (i.e. numbers) and replacing it with * in your search terms. (This does not apply to obfuscated internal minecraft function names such as func_78471_a. These are safe to google in quotes, since they are always the same within a given minecraft version)
    • For example, if your error looks like this, you might Google: minecraft advanced rocketry "NullPointerException: Unexpected error" "RenderPlanetarySky" Note where I took keywords from in the original log.
    • If the only result is a pastebin or gist link, then google that URL to find the actual discussion.
    • Generally, you're looking for issue trackers and forum posts. If a result is from github.com, forum.feed-the-beast.com, or reddit.com, there is a good chance it has a solution or explanation. If an issue tracker seems to indicate it's a bug that's since been fixed, then try updating that mod and its dependencies (backup your pack and world before trying this).
      • Know that not everyone in random forums know what they're talking about. You should try advice given in forums, but it might not end up fixing the problem, or might just be a troubleshooting step. Be wary of downloads, and generally practice safe browsing, including by using an ad blocker. VirusTotal is a website that can be used to check for malware, and the StopModReposts extension can check for malicious minecraft downloads.
    • Stack Overflow links won't be as useful, as they'll likely just provide generic information about a java error in the context of a totally different program. More so useful for mod developers. (No, this does not mean you should spam bug trackers with Stack Overflow links. Mod developers are well aware of how to find information on error messages)
    • If a result seems to have useful information but no solution provided works, add a link to it in your post here.
  • Tell us what you've tried, and whether or not the crash report changed after trying it. Include any important crash reports and logs.
  • Does the crash occur in a newly generated world? If yes, what about a superflat map?
  • Does the crash happen with other modpacks?
  • Does the crash occur in vanilla, and/or in an empty pack with just forge?
  • Without Optifine?
  • Without Foamfix, BetterFPS, Fastcraft, shaders, etc.?
  • Different Java version? Oracle JRE instead of OpenJDK, or vice versa?
  • After you update your operating system and other software packages, and reboot?
  • You may just need to clean up malware or incompatible software. If you've recently downloaded anything from Softonic or CNET download.com, there is a very high likelihood th