What creeps me out are coworkers who have intertwined personal lives outside of work. I'm not talking about intimate relationships but social stuff, like shopping for wedding dresses together, having drinks together regularly, going on weekend outings together, having lunch with departed supervisors, knowing everything about each other's lives, etc. It always shocks me when I learn about such things. I think that is just bizarre.
On the same line, I think hugs from coworkers are pretty creepy too. I like hugs, but I prefer to get them from people I want to hug.