Replaced catalytic converter

I took my car in and had the catalytic converter replaced with a exact fit 3 bolt catalytic converter. I walked into the bay just as the job was being completed, The mechanic lowered the car and turned it on; the first thing I noticed was that the car was louder than normal so he raised the car back up to see what was causing the loudness and all of a sudden a fluid started leaking from the front of the car. It was fuel from the injectors. Their most experienced mechanic had left so there was no one there who knew what could have caused the problem and they were about to close. So I left the car there. Any insight on what could have caused the problem. Note: the old catalytic converter was showing signs of being clogged, that's why I had it replaced.