| I'm assuming you're using some version of OS X, but you don't specify this (the 512E reference might indicate you're using OS 9 or earlier). So the following assumes OS X. You also don't indicate what you're seeing as a problem: Is it a problem that the backup files are being created and you don't know why, or is it a problem that the backup files contain the unknown characters but you know what program is creating them? Again, I'll assume the former is the case and you don't know where the files are coming from.
Do the file names start with #X and look something like this: #x00ac? If so, they may be hex numbers representing a Unicode character. That could indicate either that there are file names with Unicode characters in them that somehow have been translated to ASCII representations of the Unicode, or that an application is hashing the file names for some reason (you might see this with an application that creates long date-stamped names and references them indirectly through a hash table, for example).
In any case, these files are most likely showing up for one of two reasons:
1. There's some backup setting in the applications (Office, Illustrator) thats generating them. Check the Options and Preferences in the apps (I use Neooffice and iWork and other graphics programs than Illustrator, so can't help you there).
2. You're running some backup program that's backing up your files for you. Do you use Time Machine? I don't, so I can't tell you how it stores backup files by name. There are many other backup programs out there. Have you downloaded any backup programs to try and forgotten to uninstall them, perhaps?
Those are the only reasons I can think of for the behavior you're observing. | 
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