After a few online videos' is where he went wrong. Welding a propane tank isn't like fixing a fence post. Those things can blow up if there's even a trace of gas left. You need to purge them completely or you're playing with fire. He's lucky he only got a hole and not a trip to the hospital.
Yeah @bettywright hit the nail on the head. Thats the part people skip - you gotta purge it with water or an inert gas first, not just let it air out. Even a tiny spark with leftover gas inside and youre done.