Potlatch 2/Using Flash Debug Player
Flash Debug Player is the version of the Flash Player used by Potlatch 2's developers (and Flash developers the world over). It pops up an alert whenever an error is encountered.
The best way to submit a bug report is to provide steps to reproduce. In other words, if you can come up with a clear set of instructions that will trigger the bug every time, that's great.
Using Flash Debug Player
But if you'd like to help the Potlatch 2 developers even more, and you're fairly comfortable poking around with the innards of your system, you can also help by using the Flash Debug Player.
Download and install the Flash Player Plugin content debugger from this page at Adobe.com. Restart your browser when required and go back to P2.
If an error occurs, an alert box will pop up with a long description of what was happening (a "stack trace"). Copy and paste this, and log it as a trac ticket (or send it to your friendly local Potlatch developer if you know them).
Logging
You can also view Potlatch 2's debugging log, which might possibly provide more information. (There's probably no need to do this unless it's expressly suggested by a developer.) The instructions on how to turn it on and view the log vary from system to system, but try [1], [2], and [3].
In particular, if you ever get an Error #2032 (preventing saving), the offending changeset will be written to the log. You can copy and paste this and, again, send it to the Potlatch developers (via trac or whatever) so they can look for any problems in it.