First check that your bug is not present in the known bugs list . If it's not there, then you should report it. The more precise the information you can give, the easier it will be to make use of your report. The list below may seem very long. This is due to how computers work.
Logic is sensitive to small errors.
You should try to include comments in as many of the categories below as possible. Remove what is unused. A text version which you can just copy and edit is here . Information on modified code, on input and on whatever else is needed to reproduce the bug. If the bug does not reproduce, e.g. when you log out completely and try again, I probably won't be able to reproduce it either.