Most restaurant/bar specific POS systems are water resistant for exactly that reason. I'm sure it that's happened though, that's why it's "water resistant" and not "water proof".

So lets say it happens to your iPad POS and you run down to the Apple store to get a new one. What happens to your data? Is it stored on the device, or is the backend hosted somewhere? How do you get your old data into your new iPad?

Another thing I just thought of... lets say there's a horrible bug that's causing some big problem. Wouldn't you have to submit a new version to Apple, which then needs to get approved by them?

All that being said, there's risks with any POS system since it's pretty much mission critical. When I launched my company and did a Show HN, some people thought it was a bad idea (it's web-based). There's a lot of what ifs you can bring up that all sound terrible to a business owner. I'm sure a lot of people will go with iPad POS systems, I'm just wondering if there's a good reason to or if it's just the cool thing to do.