Probably the biggest problem is that documentation doesn't exist, is wrong, or is the crappy "GetItems returns Items" variety. POS companies will often charge

Probably the biggest problem is that documentation doesn't exist, is wrong, or is the crappy "GetItems returns Items" variety. POS companies will often charge you a large sum of money in order to work with their systems. I think we paid Micros $10k to be able to integrate with their junk, and we had to fly to Maryland and sit in their offices for 2 days while they ran around being useless. Workflows can be vastly different. If you're trying to write an IPOS interface that abstracts all that pain away for the client that's interacting w/ the system, it can lead to weird situations. Some POS systems will let you open a check w/ nothing on it (Aloha) but some won't (Micros). And on Aloha, if a server logs in and looks at that empty check before you put something on it, it'll automatically close. There are a ton of edge cases that you have to test, and the way two POS systems work in similar conditions are never the same. Getting menu data out of a POS is a nightmare, as there's usually a ton of unused junk that you don't need. Often there are many duplicates, so the Onion topping you add to a taco is different from the Onion topping you add to nachos, and both are different from the Onion topping you'd put on a burrito. Despite the fact that it's literally the same onion being used. Sometimes, menu items that are on sale for happy hour or something aren't different items, but they get modified with "happy hour". You don't want your users seeing that junk, so you have to do a bunch of magic behind the scenes to submit the normal item AND the happy hour mod when the user orders it. I could go on, there's a LOT of this weirdness when working with POS systems and most of the time you don't know what it'll be until you've sat there banging your head against your desk because when you order a Sandwich + fries it works, but a Sandwich + garlic fries will only add the fries to the cart. Feels good when you're finally done with the integration, but it's hell getting there. Oh, and a lot of the time there is no real API, you're changing database files or doing some sort of dark voodoo magic to get the thing to work. Sometimes you get lucky and there's a SOAP service or some COM calls you can make that'll play nice with the POS, but that's hardly ever the case.