For me when I export a clip that's long, say 60mins or so, FCP will occasionally fail to complete writing the file name.
For example if the clip you're exporting from FCP is supposed to be called
"A Long Clip_Version 01" which should appear as
"A Long Clip_Version 01.mov" when exported, it'll sometimes end up as
"A Long Clip_Versi#WTF.mov".
It seems more likely to happen with longer file names as well. The good news is that the file is likely 100% A-OK. The bad news is you have to rename it to what you want. Meh.
I suspect that it has something to do with temporary file names and how the Finder works. When the Finder is working with a file (actually mainly creating a file) it'll will tend to assign a semi-random name to it until it is done with the file. This is especially true if the Finder is replacing a file with a new one.
You can see this in action yourself with a tool like fseventer. There are many reasons why the Finder does this little bit of prestidigitation but one specific example (but not a hard rule) is if you change the preferences to something, the Finder won't always write the changes to the currently in-use pref file but it'll create a new one, with a random name or portion of a name, and when it's done will quickly trash the file in use and name the new one, the old name. The app using the prefs never knows the difference.