INSTALL JAVA 7 OR HIGHER!

[SanAndreaP]

If you have this crash:
java.lang.UnsupportedClassVersionError: [...] : Unsupported major.minor version 51.0

Then you have an outdated Java (most likely Java 6).

If you're on Windows or Linux (although I never had complaints on either about this), UPDATE! It's not that hard and there are plenty of tutorials out there. Minimum required is Java 7, recommended for you is Java 8, though.
Or, for Windows, get the new launcher from Mojang which automatically ships Java 8.

If you're on Mac, it's trickier, since Mac ships Java automatically (also the Mac launcher for Minecraft seems to force using its Java 6 version it ships). Most new Macs ship at least Java 7 or higher, but old ones still run on Java 6.
I recommend to get the new launcher (updating your java is still required on your end, though): https://twitter.com/Dinnerbone/status/642340216636882944
If that doesn't work, read on:
So if you have an older Mac, or you have this problem even on a newer Mac, here are some resources on how to install and use Java 7/8 with Minecraft:
http://www.minecraftforum.net/forums/mapping-and-modding/minecraft-tools/2167922-jre-list-tool-switching-to-java-7-on-osx
This might be helpful as well:
http://www.minecraftforum.net/forums/minecraft-discussion/discussion/2247090-tutorial-how-to-use-java-7-on-mac
I dunno when this happens, but if it's available for Mac, you can also get the new launcher from Mojang which automatically ships Java 8.

Also make sure you run the latest Forge for 1.7.10!

DO NOT open new issues IF your crash/log contains the aforementioned line! If you do, I will auto-close your issue and reference you here!