my only example
of this was bad. I was at a midwest casino playing casually while on a business trip. Crowded shoe games. At a couple of points where I had increased my bet significantly, someone behind me added $1000 to the amount I had in the circle ($200 at the time). I looked at the dealer, never having encountered this before and he said "if you don't object, we allow this when the tables are crowded..." We played out the hand and he won and with the count still + he tossed out another $1000 bet. He was up a couple of grand pretty quick, I was up about $800 for the 45 minutes I played, when I was asked to step away from the table.
Bottom line was we were accused of being a "team" and were told to play anything but 21. Never had seen the guy before, but we had a good laugh and left separately.
So there is/was some risk involved that I was unaware of.