A test harness may cause problems for the lab controller if it submits excessively large requests to the proxy. The standard test harness works around this by breaking up large requests into smaller chunks, but a custom test harness may not do that. A configurable hard size limit should be enforced by the lab controller so that custom test harnesses can be forced to behave themselves.
https://bugzilla.redhat.com/show_bug.cgi?id=537479 is potentially related