Apache :: Seemingly Legit Requests Generating 400 Bad Request Errors

So I've got a problem where a small percentage of incoming requests are resulting in "400 bad request" errors and I could really use some input. At first I thought they were just caused by malicious spiders, scrapers, etc. but they seem to be legitimate requests.

I'm running Apache 2.2.15 and mod_perl2.

The first thing I did was turn on mod_logio and interestingly enough, for every request where this happens the request headers are between 8000-9000 bytes, whereas with most requests it's under 1000. Hmm.

There are a lot of cookies being set, and it's happening across all browsers and operating systems, so I assumed it had to be related to bad or "corrupted" cookies somehow - but it's not.

I added "%{Cookie}i" to my LogFormat directive hoping that would provide some clues, but as it turns out half the time the 400 error is returned the client doesn't even have a cookie. Darn.

Next I fired up mod_log_forensic hoping to be able to see ALL the request headers, but as luck would have it nothing is logged when it happens. I guess Apache is returning the 400 error before the forensic module gets to do its logging?

By the way, when this happens I see this in the error log:

request failed: error reading the headers

To me this says Apache doesn't like something about the raw incoming request, rather than a problem with our rewriting, etc. Or am I misunderstanding the error?

I'm at a loss where to go from here. Is there some other way that I can easily see all the request headers? I feel like that's the only thing that will possibly provide a clue as to what's going on.