Are there any specific examples of backward incompatibilities between Java versions?

Question

Have there been incompatibilities between Java releases where Java source code/Java class files targeting Java version X won\'t compile/run under version Y (where Y > X) ?

Answer 1

The semantics of the memory model changed from 1.4 to 1.5. It was changed to allow besides other things double checked locking again. (I think volatile semantics were fixed.) It was broken.