Was there ever a "breaking change" in the Java language specification?

Question

With the problably widely known exception of the introduction of the 'assert' keyword, has there ever been a change in the Java language specification which caused old code to be no longer compatible with newer source levels of the JDK?

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Summary so far (many thanks for the comments):

"Older" Java code can cause compilation errors, when upgrading to a later version, if the code uses declarations using one of the keywords which where introduced in a later version of the Java language specification (JLS):

• assert
• enum
• strictfp

Answer 1

Java 1.2

• strictfp keyword.

Java 1.4

• assert keyword.

Java 1.5

• enum keyword.

Answer 2

I believe that _ was designated a "keyword" in Java 9. See https://blogs.oracle.com/sundararajan/underscore-is-a-keyword-in-java-9%2c-use-this-script-to-check-your-code

Answer 3

Java 10 made var a keyword. It's still a valid variable name but an illegal class name.