The Forge is the name of PrestaShop's bugtracker, which uses Atlassian's JIRA tool. This is the best place to describe bugs you are witnessing on your PrestaShop installation, and have the developers try to fix it.
In order to participate, you need to create an account to use the bug tracker. You can do it here: http://forge.prestashop.com/secure/Signup\!default.jspa
Fill all fields and validate your account request.
Once your account is validated, click here to log in: http://forge.prestashop.com/secure.
You can now create your first issue:
You now can open a new ticket:
On the next screen, fill all information to create your issue.
When writing a bug report, please read and apply Mozilla's own guidelines: https://developer.mozilla.org/en-US/docs/Mozilla/QA/Bug_writing_guidelines