We tried adding a new priority following the instructions written out in http://fogbugz.stackexchange.com/questions/2162/can-i-add-additional-priorities-to-fogbugz-closed.
Our goal was to create a priority even more critical than 1 (because over the course of several years, P1 has lost some of its meaning). We decided to make a P0.
However, after going through the steps in the link above, we're finding we can't actually set any of our bugs to the new 0 priority. It shows up in the field as a selectable value, but trying to actually set it just keeps the original priority.
It might be relevant to note that the "default" option is tied to this functionality somewhat. First, new bugs will appear to have P0 as default even though it's set to P3. Upon submission, it's actually correctly set to P3. If P0 is the actual default and one creates a new bug, we can successfully set a bug to P0 that way. That is the only way we can create a P0 bug.
Any ideas on what might be going wrong?