The tailshaft on my Jeep was leaking a little and I wanted to replace the O-ring to stop the leak. I pulled out the drive shaft and was trying to get the seal our of the tail shaft. I broke a dental pick trying to get the darn thing out, using more and more force. Out comes the screwdriver. Eventually, I pry out the O-ring, not realizing there was an aluminum lip behind it to keep it from falling into the transmission case. So I had pried out the O-ring, *and* broke off a piece of the lip and removed that too.
$78 later ($72 for a new tailshaft alone) I had a new setup under the Jeep. Fixed the leak though.