Forgot to mention that we're using Akamai, and it's actually Akamai's servers that are hitting the limit. We'll see if we'll be able to convince them to bump up the limit. I have a hunch that there might be some resistance because of performance implications (I'm not an operations guy, my knowledge of how web servers work internally is limited). On the other hand, it seemed to intermittently work, so there may be some servers in their farm that are configured differently. Or it could have been due to fluctuations in the header length because of different query string lengths and cookie changes.