I think, I have found an error in Apache::GTopLimit 1.10:
return OK if
(++$Apache::GTopLimit::REQUEST_COUNT < $Apache::GTopLimit::CHECK_EVERY_N_REQUESTS);
$Apache::GTopLimit::REQUEST_COUNT = 1;
Should be 0, not 1. (or change to
$Apache::GTopLimit::REQUEST_COUNT++).
Because on second request $Apache::GTopLimit::REQUEST_COUNT will be 2.
If $Apache::GTopLimit::CHECK_EVERY_N_REQUESTS =2, then it will
proceed on every request, not every other time.
Logical explanation: current request is already counted, so next
request should be number 1.