Important note that bears repeating about iXtreme LT+ v1.91 (and this probably applies to any subsequent firmware as long as additional random AP25 challenges [nicknamed "AP26"] are being used):
"Due to the new AP26 random challenges the safety freeze for unknown or bad AP25 data is no longer supported. Please ensure your AP25 data is correct for the dashboard version you are using."
This means that (for consoles which do AP25 checks [see the AP25 FAQ for info]):
AP25 backups (see the list of AP25 games) that are not patched with AP25 replay data will fail the normal AP25 challenges outright. (The normal AP25 challenges from the DAE table are still being performed along with the new random "AP26" challenges.)
AP25 backups that are patched with old/incomplete/bogus AP25 replay data will fail the normal AP25 challenges outright.
AP25 backups that are patched with current AP25 replay data will fail the normal AP25 challenges outright when the DAE table is updated again. So far the DAE table has only been updated through system (dashboard) updates but it could possibly be updated by other means.
Any of these failures will probably flag your console for a ban in the next banwave. As always you should be very wary of system updates and carefully follow the news here as well as on team-xecuter.com and xbox-scene.com.