
Great thread.
Czech (mostly Prague) ones I know about:
Prague Zoo has the "without bars" image so as someone mentioned above only dangerous animals mustn't escape at all cost. There are free reanging animals like peacocks, even wild population of some animals within the area of the zoos (ducks are the most visible of them). Some reared youngster of kept species are free to leave (like storks). Aside from that I read in 90's some small specie of monkeys (sorry I can't remeber which one) escaped (probably the whole group) from !no bars" exibit. Unluckilly the zoo couldn't capture them before winter and they didn't make it to the next spring.
Barbary macaques (in mixed exhibit with barbary sheep) found out that some bars around their enclosure are not sufficient to keep them in...
YouTube - Monkeys Escape from Prague Zoo ...that happened a few times, the most escaping one was sent to more traditional caged enclosure in other zoo (Bratislava I think). I noticed recently that one individual tried to do something simillar somewhere else in the same enclosure but it was too big to do it succesfully. Memories, memories
Once, a pig tailed macaque somehow entered komodo dragon exhibit in Indonesian Jungle pavilon
YouTube - Monkey vs Komodo ... strange stuff.
During floods some escapes occured.
In big mammal pavilon, a female hippo swam into elephants outdoor exhibit and had to be shot once the water was as high as the fences. A male elephant had to be shot before he would had drowned (the other option was let him free). A rifle man had to spent a night on the top of the roof of the pavilon just in case before two other hippos were still inside (a young female drowned but her father survived).
From sea lion group (1.3 - few weeks old male was evacuated just in case before - the zoo got wrong information and couldn't evacuate in time) one female just swam around flooded zoo and was captured (keepers realized later she was pregnant), two other females swam away and was saved in distant cities along the river and Gaston, the male, made it to Germany and was captured somewhere near Dresden. Stressed, in poor condition, he died on the way back home. Others survived and a female sired by Gaston was borned months later (keepers weren't aware of it, they thought that even if the female was pregnant she would probably lost it).
During my last visit I noticed a brown lemur without a front leg in the walkthrough enclosure. I chatted with a keeper a little bit and apparently this individual oneday in the past escaped from its (now former) enclosure but escaped into bat eared foxes' exhibit and was severely injured, recovered but the leg couldn't be saved.
Not only escapes from zoo happened unluckilly - two hungry dogs escaped from a squat few kilometers away from zoo, digged into the zoo. Twice. Although the secuirity responded they killed some goats and sheeps from children zoo and wounded other. After the second attack the dogs were confiscated from their owner. They were evidently in bad state and hungry.
In Dvůr there was an electricity collapse once and a gorilla male nearly escaped because of that. During the danger Prague gorilla keeper who was working there by this time, saw a mother with a baby so he cathed the baby and run away from there. The mother started screaming (not knowing what's going on) thinking the keeper is trying to steal her baby

I don't remeber details I think the male gorilla just went back to the indoor part of his exhibit or something like that - simply nothing happened.