I just ran a test of the pipestem problem, and I haven't been able to reproduce it.
Does your forum have many hacks installed? Perhaps the "Last Person To Register Hack" or a hack similar in nature which displays the user name on the main page contains a bug.
Just registering a user with one pipestem in the nick doesn't seem to have any adverse affects at all.
Donate to UBBDev today to help aid in Operational, Server and Script Maintenance, and Development costs.
Please also see our parent organization VNC Web Services if you're in the need of a new UBB.threads Install or Upgrade, Site/Server Migrations, or Security and Coding Services.