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

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) ?

By "Java release" I mean versions such as:

• JDK 1.0 (January, 1996)
• JDK 1.1 (February, 1997)
• J2SE 1.2 (December, 1998)
• J2SE 1.3 (May, 2000)
• J2SE 1.4 (February, 2002)
• J2SE 5.0 (September, 2004)
• Java SE 6 (December, 2006)

House rules:

• Please include references and code examples where possible.
• Please try to be very specific/concrete in your answer.
• A class that is being marked as @Deprecated does not count as a backwards incompatibility.

Compatibility notes for various versions:

• Java 1.4
• Java 5
• Java 6
• Java 7
• Java 8

The first major hiccup I remember was the introduction of assert in Java 1.4. It affected a lot of JUnit code:

11. In Java 1.4, assert is a keyword. Won't this conflict with JUnit's assert() method?

JUnit 3.7 deprecated assert() and replaced it with assertTrue(), which works exactly the same way.

JUnit 4 is compatible with the assert keyword. If you run with the -ea JVM switch, assertions that fail will be reported by JUnit.