NOTE: I'm the author of RegexKitLite.
This is an internal assertion failure within RegexKitLite. There are a lot of built-in internal assertion checks. This one is saying that after retrieving a cached compiled regex from the cache, the retrieved cached regex is not set up exactly as it should be for some reason.
The best thing you can do is submit a bug report at sourceforge.net RegexKit bug tracker. If at all possible, submit a test case that reproduces the bug. This is a total, wild, out of the blue guess, but based on the assertion message, I'd bet that it has something to do with match operations in which you are using the range:
parameter, and that range is constantly 'moving" and shrinking. That range may cross the small/large buffer size, somewhere around 2048 characters. Another possibility is searching strings that contain Unicode characters and RegexKitLite is using the strings direct buffer, possibly on a mutable string and the backing buffer has be reallocated because it grew or shrank... or, a mutable string that was "non-Unicode" has been mutated and now contains Unicode characters, and it has switch from the cached UTF-16 conversion to the strings direct buffer.
Hopefully, that will help you narrow down the corner case that is triggering the assertion failure. Hey, at least it's detecting a problem instead of plodding along and seg faulting or returning bogus result strings. :)