I think that it happens less frequently these days than it used to in the past, but there are times when Illustrator files could become corrupt. Prior to Illustrator 9, Illustrator's native file format was PostScript and therefore, it was always possible to open a file in a text editor and delete or fix the code that was causing the issue. However, since Illustrator 9, the file format is now PDF based and that means such editing isn't possible...
Unless, that is, you know the secret hidden feature that Paul George told me about...
1) Open the preferences file and look for the entry named "enableContentRecovery", change the value from 0 to 1. This turns on the feature.
2) Launch Illustrator and choose "File>Open". Select the file from which you wish to recover the roundtrip information.
3) Click "OK" while holding down the command, shift and option keys (control, shift and alt on Windows).
Following this procedure will result in a new empty document in Illustrator. This you should just close without saving. In addition the roundtrip information from the file is extracted to a new file in the same directory as the original. The new file is given the same name as the original preceded by an underscore character.
Once the roundtrip information has been extracted it can be viewed in an editor and any problems corrected. The corrected file can then be opened in Illustrator. Note that images in the file are saved as binary. Therefore the editor used to modify the file must be able to preserve binary data.