I seriously doubt that trashing the Prefs is the correct prognosis for this issue. It's a little naive to suggest trashing prefs as a catch-all for solving every error. But for the record, if you want to trash the prefs, you don't have to do it manually. From the manual:
Resetting preferences can be helpful if you are having problems with the application.
Press and hold Alt+Control+Shift (Windows) or Option+Command+Shift (Mac OS) as you start Illustrator. Your current settings are deleted.
Remove or rename the AIPrefs file (Windows) or Adobe Illustrator Prefs file (Mac OS). New preferences files are created the next time you start Illustrator.
But in this case resetting the preferences is barking up the wrong tree, I'm afraid. The issue is in the file, not the application, so prefs don't come into the equation. You get this for two reasons, it appears:
1. an issue with rasterized artwork in Illustrator (begging the question - what are you doing with rasterized artwork in AI in the first place, but I digress)
2. a geometry issue - most often this comes from one or multiple Pathfinder operations, or working with illegal / poorly constructed geometry coming in from another source (a font, or EPS file)
In the current situation I'm experiencing, it's on an outlined font (the letter "L" with no compound path) that I'm trying to cut manually with the Scissors tool. Once I perform the cut, the error dialog appears, saying "The operation cannot complete because of an unknown error [!IMP]". Following that is a second modal dialog saying "An error occurred while processing the appearance of an object".
Because this is a geometry issue, copying and pasting the artwork into a new, blank document will not do anything toward mitigating the issue, as the erroneous geometry comes along with the artwork that's pasted.
Retracing / re-drawing is certainly a workaround, but certainly not the most desirable solution.
The old Offset Path at 0px offset doesn't solve the problem (also tested on larger offsets and that did nothing as well).
Strangely, the Knife tool did not cause the same error. In my specific case, this will allow me to proceed, but now my curiosity is piqued.
Following up on the Knife tool, is it possible it doesn't like the unclosed path? So I cut the opposite side of the path to completely separate the object (same error). This has caused a Compound Path to be created (the object was NOT a compound path prior to this). Release Compound path (still error). Now close the two shapes. No more error.
So there's something in the geometry (quite likely came in with the Font) that had some kind of embedded closed / compound shape that doesn't actually exist and is causing the parser to choke.
I'm out of ideas for how to "clean" the shape. Simplify errors out and things just get weird from there. Overlaying a large solid box and using Pathfinder > Divide and extracting the "hole" out of the solid still has the same geometry error. So I don't think there's any programmatic way to remove the geometry error.
Hi there Joe.
Happens the same to me and it's no good to 'get use to it'.
In my case, each time I press ctrl+Y to see my canvas outlined the error appears.
I founded I got a shape sliced with the knife tool, and that generates lot of anchor points data and make illustrator freak out.
To discover wich object is bodering, start copying by parts your canvas to a blank document and checking in the blank doc wich part have issues.
Hope I helped : )