A couple of really common spots I find where this happens fairly often:
Sebilis Disco:
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Pets/mobs being pulled through this corner have a chance of falling through the floor. Can be avoided by walking mobs through the door - however all you need is one add while pulling for fun times to start.
Sebilis Crypt/Emp:
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Pets/mobs being pulled through this corner have a chance of falling through the floor.
I cannot comment on other zones but I know this did start the patch last year. I had spent a significant amount of time in Seb before and after and noticed it almost straight away.