After spending hours with Adobe support remotely trying to fix this issue, they tried to tell me that my file was just corrupted and that I would have to start my project all over again. I later found out that the error was being caused by a pattern in my document that was scaled down to -1%. Once I adjusted the scale, my document saved as normal. Hope this helps!

I've found that one issue that causes this is when the permissions for certain fonts are not allowed for the user account being used on that computer.  A clue that this is the problem is that if you Create Outlines from all fonts and then the file will save. Another clue is that when saving there might be a warning sign at the bottom of the save version screen that says "only fonts with permissions will be embedded" or something similar. 

Solution:   I went into the fonts folder on my computer  C: drive > Windows > Fonts,  found the font that was being used in the project. Right click on it, select Properties from popup menu. Click on Security tab, then make sure that all user accounts have Full Control selected ( check box ), then click Apply.  That took care of the problem permanently for me when we used that font. One one computer I have AI set to run as Administrator and I have all permissions for Admin set to Full Control. That computer has never had this issue. The issue has been on computers used by employees who don't have full permissions on everything.  So I am looking for a way to make just the entire listing of fonts have Full Control set in the Permissions area, without giving full access as Admin to them.

I'm about three years late, but I hope this helps someone else.  I ran into this error message when opening a pdf made by an architect and after some digging and trial and error I found elements called 'non-native art' in the layers panel which is where the culprit was.  I deleted all of them and the file save was successful.

I had the same problem with a completely new file.
I've never seen this error before. I had pasted in several pages of a pdf of scanned sketches of characters I need to trace. I rarely place pdf's in images, so I thought that might be the cause, also because there was nothing else in the file except a couple of artboards which I renamed for when I need to export the files..
The artboards couldn't be the problem as I tried inserting the same pdfs into a new file with only one artboad

I couldn't seem to save the file no matter what version of Illustrator I save in.

When I read some of the posts in here, especially the ones about memory, I thought it might be because it links to the file externally or because the scans have some resolution problems as is sometimes the case with InDesign files with pdf or large images placed in them.

The solution I find to work was simply to embed all the pdf pages in the illustrator document, after that I had no trouble saving the document, at least not in the newest version of Illustrator (CC 2015)

i dnt think so,cz i faced the same problem,the cs6 file will save if u have a blank document,bt when u put data and wanna save it shows error

Yeah this all started for me after upgrading from Win7 to Win10. If I open Illustrator CS3 and save as the empty document it saves just fine. If I do anything at all...I mean draw one shape and then save I get this "An unknown error" has occurred. Doesn't matter what's checked or unchecked, I always get the error. This seems really odd after this program working flawlessly for so many years.

POSSIBLE SOLUTION:

This has worked for me several times, don't know if it will solve it for all of you:

1. Make sure all objects are unlocked.

2. Select all objects and Object>expand (fill and stroke)

3. Try to save again. If you are still unable to save. Create a new file and try to copy and paste all of your new expanded objects into the new file.

4. If still nothing working, try exporting the project as an editable PDF. Restart and open the PDF, then save as Illustrator.

I found that part of the reason my files weren't saving is I had a mixture of expanded and non expanded objects. When I expanded them all it allowed me to save the files.

WHY CAN'T ANYONE IN THE WHOLE. WIDE. WORLD..........FIGURE THIS STUPID THING OUT!!!

C'MON............... I. HAVE. CUSTOMERS. WAITING!!!!!!!!!!!!!!!!!!!!!!!! DOES ANYBODY OUT THERE

GET IT????? SHEESH

I found a solution if anyone still needs help with this issue. Select all your text layers, Click Type--> Create Outlines. Problem solved.

(Make sure you don't need to change any text before you do this because it makes a path out of the text layer)

This is exactly whats happening to me can someone help, why when I open up my work on illustrator " An unknown error has occurred" appears. Is this because I use the transparency on my images?