Here's an odd one. I have a GFCI-protected circuit with a Uninterruptible Power Supply on it. I've found that when I test the outlets on the circuit with a GFCI outlet tester the tester does not trip the GFCI. However, when I unplug the UPS from the circuit the tester trips the GFCI as expected. Why would this be? The UPS is a Minuteman Entrust 500VA in case it matters.
Note: I'm am not plugging the tester into the UPS, I am plugging it into either of the 2 "regular outlets" or the actual GFCI outlet.