Report it to the project's bug tracker system (probably easy to find on its website).
Document it to make it a feature!
Let the developers know, and include instructions and a test-case if possible. Written almost 14 years ago, and still true today: http://www.chiark.greenend.org.uk/~sgtatham/bugs.html
You can see if anyone else has found it and how they dealt with it. If not, then you can report a new bug and help the developers.
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