There is not any information in your post about would be causing that error.
Questions that may help track down the problem:
What layout is the user on when the error occurs?
Is there any script triggers on this layout?
What is the user doing when the error occurs? such are they click a print button, email, button or scrolling records?
Are there two or more users trying to access the same record(s)? FM locks records, so this may be two users trying to run the same type report or list with the same records.
After answering those questions it will give you an idea what scripts are running, which may lead to the cause of the error.
Thanks! Yea, I need to try to track down how it is triggered. If I get more info I'm come back.