HELP - Can’t open the illustration. The illustration contains an illegal operand?

Ouch – that's a case of bad luck !!

Something must have messed up the file's structure or content, like Jacob also explains...

Please don't throw away the file, because if someone spots a culprit in the message you reported, then you might be able to open the file in a text editor (since the PostScript behind the scenes is in fact a text based language) and correct it. But we need a PostScript specialist to help you out.

In the meantime, do try to Place the file into Illustrator or even InDesign, and save it as a new Illustrator file or a PDF. Such procedure sometimes strips an offending but useless element. (Many elements in an Illustrator file are for backwards compatibility or reference purposes only, and not crucial for immediate viewing or printing. Think of unused swatches, symbols, patterns, anything.)

Opening this new file back into Illustrator ill probably yield a number of extra clipping masks, to let the illustration be restricted by the surrounding bounding box and even a page box. So you might need to release and delete some of the masks to make the content directly feasible again (in stead of working in isolation modes).

Cooper,

The message means that the file has been corrupted.

If someone recognizes the actual error in the code, it may be quite easy.

One thing often tried first is to create a new document and File>Place the (PDF contents, if any, of the) corrupted one to see how much may be rescued that way.

Here are some websites where you can see whether it can rescue the actual file, and if it can, you may pay for a subscription to have it done,

http://www.recoverytoolbox.com/buy_illustrator.html

http://markzware.com/adobe-software/fix-illustrator-file-unknown-error-occurred-pdf2dtp-file-recovery/

http://www.illustrator.fixtoolbox.com/

http://www.erepairillustrator.com/

As far as I remember, the first one is for Win and the second one is for Mac, while the third and fourth one should be for both.

Here are a few pages about struggling with it yourself, or maybe share crucial parts of the code and hope that someone recognizes the actual error in the code, in which case it may be quite easy.:

http://daxxter.wordpress.com/2009/04/16/how-to-recover-a-corrupted-illustrator-ai-file/

http://helpx.adobe.com/illustrator/kb/troubleshoot-damaged-illustrator-files.html

http://kb2.adobe.com/cps/500/cpsid_50032.html

http://kb2.adobe.com/cps/500/cpsid_50031.html

http://helpx.adobe.com/illustrator/kb/enable-content-recovery-mode-illustrator.html

There are also more suggestions here, on this partially overlapping help page:

https://helpx.adobe.com/illustrator/kb/troubleshoot-damaged-illustrator-files.html

It is always recommended to create backups with different names regularly, and to store backups in more than one place/way.

And also to save often.

If you have a CC subscription, saving files in your Creative Cloud Files folder will automatically create and keep versions up to 10 days.

https://helpx.adobe.com/illustrator/using/crash-file-data-recovery.html

https://helpx.adobe.com/creative-cloud/help/sync-files.html

https://helpx.adobe.com/creative-cloud/help/versioning-faq.html

and here are some other suggestions,

https://forums.adobe.com/message/9328988#9328988