I have 6 of these 5165/5166 switches on my layout, and all are powered by fixed voltage plugs. The short circuit occurred only on the one 5166 that I was replacing the bulb...while the switch had the power on. None of the other switches were affected.
But I have experienced similar issues at different times with some of the other 5165/5166's on my layout. Very susceptible to shorts. when replacing the bulbs while the switch was powered up. My suggestion is to make sure you turn off the power when attempting to replace a burned out bulb.
Frankly, I'm not a big fan of these switches in that they are not the same quality as the O22's. I am pleased to learn I can swap the switch motors on these with the O22's, and that should solve the problem.