An integer literal consisting of a leading 0 digit followed by one or more digits in the range 0-7 is an octal literal. This can lead to two problems:

An octal literal can be misread by a programmer as a decimal literal.
A programmer might accidentally start a decimal literal with a zero, so that the compiler treats the decimal literal as an octal literal. For example, 010 is equal to 8, not 10.

To avoid these problems:

Avoid using octal literals so that programmers do not confuse them with decimal literals. However, if you need to use octal literals, you should add a comment to each octal literal indicating the intention to use octal literals.
When typing decimal literals, be careful not to begin them with a zero accidentally.

J. Bloch and N. Gafter, Java Puzzlers: Traps, Pitfalls, and Corner Cases, Puzzle 59. Addison-Wesley, 2005.

Java Language Specification: Integer Literals.