Visible crew/equipment: When Deck Shifflet (played by Danny DeVito) and another actor are calling their office from a payphone, in order to setup another law firm, once the call has been completed and there is some back and forth between the Danny and the other actor, a crew member is entering the scene via a side alley, more humorous is how said crew member leaves the scene, on realizing that he has just walked onto a live set, the crew member proceeds to stop and walk in reverse out of shot. (01:13:57)
Revealing mistake: During his final speech Rudy projects a video of Donny Ray on a screen. When he rolls up the screen at the end, the projection of Donny Ray's face should still be visible on the wall behind. (01:57:45)
Answer: I don't know how prevalent this is (or how it actually works), but one possibility would be that he was under contract with a particular agency (or company/ business) and could only receive credit for work performed under this contract and/or no other agency could benefit from using him. His name cannot show up in something he did outside of that contract.
KeyZOid