The STS Orbiter was infamous for losing tiles. This occurred at a higher rate than expected.
I understand the epoxy used to attach the tiles to the shuttle cured extremely quicky, so a fresh batch would have to be made every couple of tiles. It appears the enterprising workers discovered that adding saliva to the epoxy as they worked would allow them to attach more tiles before having to break off to mix another batch.
I though everything related to spacecraft construction was logged and double checked, Why wasn't this modification to procedure noticed for so long?
Bonus internet points for stats on the difference in tile shedding rates after the practice was stopped.