what are the gotchas with message queue consumers?

we're looking at putting in a message queue for how different services talk to each other, and that part seems pretty clear. but, the consumer side of things worries me a bit. what are some common mistakes or tough spots when you're setting up and running message queue consumers. are there good ways to deal with poison pills, make sure stuff only happens once (and when you even need that), and scale your consumers well. really looking for advice that actually works in practice.