jd-core | JD-Core is a JAVA decompiler written in JAVA | Plugin library

While reserved words may not occur in Java source code, they are permitted in compiled Java code. Some code obfuscation tools can make use of this to make decompiling harder. For instance, ProGuard provides the following option:

-obfuscationdictionary filename

Specifies a text file from which all valid words are used as obfuscated field and method names. By default, short names like 'a', 'b', etc. are used as obfuscated names. With an obfuscation dictionary, you can specify a list of reserved key words, or identifiers with foreign characters, for instance. White space, punctuation characters, duplicate words, and comments after a# sign are ignored. Note that an obfuscation dictionary hardly improves the obfuscation. Decent compilers can automatically replace them, and the effect can fairly simply be undone by obfuscating again with simpler names. The most useful application is specifying strings that are typically already present in class files (such as 'Code'), thus reducing the class file sizes just a little bit more. Only applicable when obfuscating.

So if you feed this a list with Java identifiers (such as this one) you end up with a class file that causes syntax errors if decompiled. Of course, you can simply rename the variables to fix these compilation errors (for instance by using ProGuard yourself before decompiling), so this is only a minor inconvenience.