im kinda mad -> i was in the dentist office getting some work done around the end of last year. I was quoted a price of $800 for a crown, in which i was told that my insurance would cover $400 of it by my dentist's assistant.
While getting this done, he says "hey we should do this second one while Im in here. do you want me to?" and while im in the chair with a tooth drilled, i asked how much for both to be sure. his assistant once again told me that it should be another $400 to bring the total to $800. I was even given the option to pay $400 that day and $400 later, but instead, i paid this full 800 that day.

Well come to find out a month later, my dental plan at the time did not cover crowns (i had actually chosen a cheaper insurance that year). so when i went to the dentist again, they were like "you still owe us $800".. am I really liable to pay that when I was quoted different prices?

Yeah, you are. There are so many insurance plans and variations on plans how can you expect the staff to know all of them? Especially when they change so fast... I don't take any big ticket medical prices for sure unless I know the provider has contacted my insurance company or I've contacted the insurance company myself (yeah, I've been known to ask schedulers to call the insurance company while I'm standing in the office...). And, of course, the paperwork you signed when you set up your account with the provider said that you would be liable for anything your insurance company didn't cover.

Always always always verify what your insurance company will pay for when you're going for big ticket stuff. I know some of it you just can't put off, but at least you can be prepared for it.