SWI-Prolog -- bug report
You found a bug if the system didn't install as described, crashes or behaves not according to its documentation or well established standards in the Prolog community.
Preferably using Bugzilla. This internet based bug-tracking system makes it possible to track bugs in an organised manner, allowed for distributed responsibility and can automatically send mail to you as the bug is confirmed, fixed, etc. In addition, it provides facilities to attach source-code to demo the problem and patches to fix the problem.
Alternatively you can send them to the SWI-Prolog mailinglist. This is somewhat less hassle, but less structured. It makes it more likely your report doesn't contain the required information, making it harder to fix your bug. The mailinglist is dedicated for `How do I ...', 'Is there a library for ...?' as well as discussing development directions for SWI-Prolog.