Two things confound me. One is simple; the other complex. Just venting, though.
The first. Why on earth do some people get confused about this auction? If you open 2NT, you play some type of Stayman and transfers, maybe more. But, for some reason a lot of people get confused when the auction is 2C...2NT or 2C...Kokish...2NT and are not sure. What the?
The other is the implied cue. I bid a suit. You bid a suit. I rebid my suit. You bid the third suit. I bid the fourth suit at the four-level. I like your second suit a lot.
Or, you transfer and bid 3NT. I bid a new suit at the four level. I have a great hand for your suit.
However, sometimes some people are mystified. What the?
There are tons more. These two just seem to have come up a lot.