A lot of the Ludum Dare games are written in Java. Much to my surprise, I did have a JRE installed on my machine. So most of the games worked out of the box, but sometimes the JRE died with a peculiar error:
Exception in thread "main" java.lang.UnsupportedClassVersionError: $file (Unsupported major.minor version 51.0)
Let's try and understand this error, shall we?