...yes, illustrator can sometimes throw unknown errors when saving, in that situation i have found you have to save out to EPS format to keep your latest amends, or as long as you save regularly, like after every satisfactory move, then i just re-open my last version...
...when the unknown error occurs, first thing your best to do is delete the preferences file before working in illustrator again...
If manually deleting this file, ensure Illustrator is closed first.
With Illustrator CS and later, it is also possible to delete the preferences automatically by holding down on application launch:
WINDOWS:
CTRL+ALT+SHIFT
MAC
Shift+Option+Command
Illustrator preferences are stored in the file "Aiprefs". This file has no file extension, is a hidden file, and its location is dependent upon the operating system version.
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WINDOWS
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To locate the file from Windows Explorer it is necessary to set Explorer Folder options > View to "View Hidden Files and Folders"
Under Win XP and Win 2K, the file will be located in:
%systemdrive% \ Documents and Settings \ ~username \ Application Data \ Adobe \ Adobe Illustrator [Version] [Settings-(CS/CSx)].
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WINDOWS VISTA
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Users\[user name]\AppData\Roaming\Adobe\Adobe Illustrator CSx Settings
(x is the version number you are using)
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Mac OSX
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Users > ~username > Library > Preferences > Adobe Illustrator CS Settings > Adobe Illustrator Prefs
...if you still encounter problems with illustrator it can also be an advantage to move the whole settings folder out, you don't have to delete it, just move it out so illustrator creates a fresh new settings folder on launch...
andrew