I have seen it happen a few times, but it does not seem to happen very often. I don't think I ever saw it happen when riding a trolley bus - usually I saw a stopped trolleybus with the driver stepping out & reatacching the collector poles before driving away. Took a couple minutes at most.

It was usually in some tricky spots - trolley wires crossing streetcar wire on Konečného náměstí (yeah, we have both :) ) or on the top of the rather steep slope of the Kotlářská street next to the Natural sciences faculty of the Masaryk university.

I guess the new trolley busses with batteries and poles thatr can be stowed automatically might be even better for this - no need to fix the poles immediately, but wait for a regular stop or even the line terminus & just use the battery.

Also once when touring one of the trolleybus depos (the small one in Husovice) the local maintenance chief mentioned that the one roundabout wired for trolley buses (yes, that exists) is a pain to maintain & maybe they could scrap it in the future, with the busses crossing it on batteries instead. :)