Modicon 258 Move instruction

I have a modicon 258 controller, and the client had an issue with a value not moving on the first scan rung. The rung had 3 move instructions in series and the middle move instruction would not execute. The tags and logic associated with the move instruction were correct as well. I replaced the Move instruction with a new move, deleted the old Move, and the system works fine. What would cause this? Is it a memory issue?

bug in software. have seen this on numerous software packages.
FB not working. then replace with the same type and then starts working again