It happened to me when I was trying to copy and modify some themes: suddenly, everything I touched was thereafter a "version incompatible with the 6.10 version of Drupal core." This is apparently quite a common problem, so I'm hoping my find will help some others out.
There are apparently a lot of reasons this error can be thrown. I think I've read through all the postings about it, seeing if I could find anything that looked like mine. I saw lots of suggestions that didn't apply to me. All I was doing was copying installed themes to a new folder. That caused them to become "incompatible." Then I discovered that I couldn't even install a new, very compatible theme like Acquia Marina -- it too became incompatible with our Drupal 6.10 core.
My problem turned out to be file permissions. My Expandrive had a custom permissions mask of User [read, write, execute], Group [read, write, execute], Other [read]. Well, the "Other" in this case is Drupal, and it couldn't execute, so it was throwing the "incompatible" error. As soon as I turned on Execute permissions for "Other," everything was fine.
If you have this problem and can't find what's causing it, and especially if something has been working just fine until recently (my situation) or is working fine on another server that you're trying to clone from, check the permissions that are being set when you upload, create, or modify a file, and make sure that Drupal can read and execute.
In fact, if you see this problem yourself, check the file permissions first, and you might not have to go any further.
Comments
This was a good catch.
I'm using symlinks in my dev environment and I didn't check the permissions on the linked directory and it's children and this was the issue. Now all is well, thank you for posting this (reminder)!
Also check config in module.info
One other possible issue could be wrong config option in module.info. Check spelling in all config options if checking permissions doesn't solve the issue