The Batcave is supposed to be one of the most secure, well hidden and safest places on the planet, a base of operations where Batman can sip tea and use his Batcomputer in peace.
But, despite his best efforts to keep the location a secret, the Batcave has been raided by at least a dozen different villains, and Batman has been forced to carve out a few new Batcaves around the city since Bane broke in.
When Bane literally broke the Bat in the Knightfall story arc he did so inside the Batcave, after breaching Wayne Manor's defenses with a frontal assault, to prove a point- Batman's
After Knightfall all kinds of characters from the Rogue's Gallery started finding their way into the Batcave including Two-Face, who simply toyed with the caretakers, and The Joker, who left Alfred short-handed.
But the strangest Batcave break-in title goes to Doctor Hugo Strange, a guy so obsessed with Batman he tried to kill him and take his place, and even built an exact replica of Wayne Manor and the Batcave as a Bat-trap.
Hugo drugged Batman and brought him into the replica, then he threw on his own Batsuit and the surreal Bat-vs-Bat battle began!
Batman got the better of Dr. Strange and actually killed him, which is when he discovered it was actually just a robotic version of the mad Dr. being remotely controlled by the real Hugo- from the real Batcave.