2ZZ-GE tore apart my clutch, pressure plate and flywheel.

2 days ago i finished my 2ZZ-GE swap, and curious thing happened as i was shifting from 3rd to 4th:

Everything went smooth with the shift, 4th engaged well and second and a half after the clutch was fully released it suddenly tried (as if i shifted Again) to overrev as if it went from 4th straight to 1st. The same millisecond it started raising the revs i kicked the clutch all the way down and released the car from gear (put it in neutral). I kicked the clutch back to the floor so fast that even the nose didn't dive into the road as if would've done if it locked in lower gear. The engine stopped and i heard pieces flying. I parked on the side of the road and tried to re-start the car. The engine wouldn't ignite. It turned like the battery was trying to die.

The following day we disassembled the car and found that a piece the size of a quite big coin was missing from the gearbox housing.

Piece of the clutch cover shattered, went straight trough the clutch disc, tore a hole in the gearbox housing and the flywheel cut trough all the bolts holding it to the crank shaft. The whole eight of them! When we separated the gear box from the engine the flywheel, clutch and the clutch cover were all in the bottom of the housing like a big salad. That's why (i guess) it didn't want to ignite the engine, since the starter didn't have full or any contact with the flywheel and didn't manage to spin the engine trough it, since it tore the bolts holding it apart.

My question is:
My reaction was astonishingly quick. I am almost 100% sure the engine didn't over-rev itself. from what it looks like the problem should be only in the flywheel-clutch department. The engine turns by hand like it should, no weird noises while turning it by hand.

Please guys, tell me there shouldn't be anything else damaged... And for Christ's sake, how the hell did that happen?!