And more significantly:

"Severus," Albus Dumbledore said, and his voice almost cracked, "do you realize what you are saying? If Harry Potter and Voldemort fight their war with Muggle weapons there will be nothing left of the world but fire!"

"What?" said Minerva. She had heard of guns, of course, but they weren't that dangerous to an experienced witch -

Severus spoke as though she weren't in the room. "Then perhaps, Headmaster, he is sending a deliberate warning to Harry Potter of exactly that; saying that any attack with Muggle weapons will be met with retaliation in kind. Command Mr. Potter to cease his use of Muggle technology in his battles; that will show him the message is received... and not give him any more ideas." Severus frowned. "Though, come to think of it, Mr. Malfoy - and of course Miss Granger - well, on second thought a blanket prohibition on technology seems wiser -"

The old wizard pressed both his hands to his forehead, and from his lips came an unsteady voice, "I begin to hope that it is Harry behind this escape... oh, Merlin defend us all, what have I done, what have I done, what will become of the world?"

There aren't really any other good candidates for what he might have done to cause this particular problem (even if he felt responsibility on account of e g. not having been able to beat Voldemort permanently himself it seems unlikely to phrase it like that).