when I was putting my L28et together I kept ripping rear main seals. I went threw about 4 or 5 of them before I realized, because it came from an auto there was that little "cap" bit on the end of the crank. tapped it with a hammer, came right off, and problem solved
and I was helping a buddy tune up his 240sx, I did all the plugs and wires and stuff up top and he did the oil. He started it up and there was a ticking off and on and I was like "WTF is that noise?" I looked under the car and come up holding a bolt and ask "dude, whats this?" ... he hadnt put the drain plug back in when he "filled" it back up with oil