When I was still in food service I got people telling me that I had to see Waiting ... pretty much all the time. The really odd thing was that a lot of customers told me I had to see the movie, which is really strange now that I've seen it.
Disclaimer! - I have never put body fluids or hair into someone's food. Buuuuuuuuuut yes I have seen food service workers participate in five-second-rule style shenanigans if a particularly awful customer is involved. What's more likely is that you'll just get shitty service and less/worse food - we know which bagels are the most stale, we know which espresso has been sitting out the longest, we know which milk might be in the process of turning and THAT'S what you get if you treat people like shit. Probably nothing that will actually make you sick, just something bad enough to discourage you from coming back. We'd also overcharge the hell out of people for being assholes. If you were cool and wanted something like a caramel macciato (not a real drink, btw, at least not the way you think it is) we'd just charge you for a latte or maybe a mocha. If you were an asshole you'd get charged for a latte, plus a vanilla shot, plus a caramel shot, plus whipped cream, plus caramel drizzle, plus a charge for an extra shot of espresso if you were particularly heinous so an approximately three dollar drink became a five-dollar-plus drink. Occasionally if you're just a terrible, reprehensible, cruel person who dehumanizes other people you're running the same risks that terrible people always run but now you're doing it with people who are paid to deal with sharp/hot things and things that you put in your body.
I once had a customer who asked me for a drink extra-hot (this lady came into my shop regularly and had a history of calling baristas liars and idiots and letting her horrid grandson run around dropping ice cream on everything then demanding another ice cream because his "fell onto the ground" and speaking to the manager if we wouldn't give him three free ice creams): I steamed the drink until it was literally boiling, which I knew because it boiled over on my hand and gave me a 2nd-degree burn, and without touching the cup (but after watching me burn myself and literally hearing me scream) she told me it wasn't hot enough and she wanted me to re-steam it before I left to ice/apply first aid to my blistering hand. So I "accidentally" knocked the cup over in her direction and it splashed off her nice leather jacket and she screamed at me and I ended up not being able to use my hand for a week and refusing her service ever after. And I had a few people tell me I should have spit in her drink instead of spilling it on her. I couldn't bring myself to do something like that to this horrid customer even after my hand had started to blister, but I know that plenty of the food service workers in that town had no such qualms, so Ryan Reynolds' line about "Rule number one, never fuck with people who handle your food" is a pretty fucking serious rule and one that I would advise everyone to take to heart.
Not every food service worker is going to drop your steak on the floor then dust it with dandruff if you're rude to them (in fact the vast majority won't) but treating servers like shit is a good way to get stale food, cold food, the wrong drink, and a long wait. And don't bring tips into this - servers expect about 15% (and have every right to - if you can't afford to tip you can't afford to eat out) but if you start treating a server like shit they realize that you're not going to tip them anyway so they might as well make sure they never want to get seated in their section or even come back to that restaurant again.
But anyway, back to the movie.
The flick is full of gross-out humor and day-to-day drama that is actually endearing and makes all of the characters look charming and lost as they muddle through their minimum wage jobs. Ryan Reynolds is in a role that's perfect for him - a good-looking asshole with no depth who makes you laugh and feel skeezy in equal measure. Justin Long is also playing a very Justin Long-y type character - they seem to have stopped making movies about affable geeks with hot girlfriends and self-awareness some time in 2011 and I miss those movies - who's likeable enough but not terribly interesting. Dane Cook is in the movie too, which is actually a huge part of why I didn't see it until nine years after its release, but don't hold him against the film - he almost doesn't register.
This is a fun, stupid, cheaply-made but well written and acted little film. It does a great job of showing what a night of food service looks like to someone stuck in that sort of job and floundering, and it reminds us all about the terrible, fun, disgusting minimum wage jobs we had when we were first learning to be grownups. The fact that it's 75% fresh from Rotten Tomatoes Audiences but only 31% from Rotten Tomatoes Critics suggests to me that either most movie critics have never had a real job or that they've never had to grow up - don't let a poor critical reaction keep you from watching this if you're looking for some sympathy for your customer-service job.