Typically, at least three of these will need to be made and gone over before final approval. If you're lucky enough to be working with film, this is all done at the lab, where special laser processes etch the titles onto the negative. But more likely, you're on video, and the same hardware used to make the check tapes is used to make the master, but you're using much more expensive tape. Problems often slip through the cracks. Lines get jumbled.
Computers crash halfway through scripts.
Spacing gets screwed up. The lab accidentally changes words. Cables go bad. The final tape always has to be triple checked, just to be on the safe side. You won't have a second chance, and any mistakes will damage your reputation.