what's easy to forget? Oh ... yeah, you're right.
... I guess I mean that sometimes it's easy to assume some deeper problem than simply, "I made a mistake". That's my personal experience anyway.
I support a solution that consist of more than a hundred files and often work with offline copies for debugging and development tasks. Since it's easy to not have every file in my offline file set, I see this a lot.
FileMaker does not lock up permanently. It's just taking a long time trying to find a file that isn't there. If I wait, I'll eventually get a "file not found" dialog where I can either manually find/select the file or I can cancel. Even better, on my windows machine, I don't have to wait for this dialog. I can press Esc to cancel the search if I don't need the file for the task at hand.
So so perhaps you did not wait long enough or something else is is wrong with the file. (I also do not get this just by entering layout mode. I have to do something more like opening a specify field dialog, the calculation dialog, or Manage|Database--all actions that cause FileMaker to pull info from every table occurrence. )