Yep, very easy to overcharge A/C system, which can damage the compressor.
He probably means he was turning the ac knob from normal ac position to max ac position and so forth.
You should have bought one that has a pressure gauge at least on it. I know my hvac buddy when he does cars, he vacs the whole system out then adds the right amount via weight. My 97gtp still has ice cold ac and has never been charged once, but my friends 07 civic si a/c system was toast because he had a hole in his condenser.