Originally Posted by SMann
The half-moon cutout groove is for temporarily holding the spring in place during re-assembly ONLY. The spring does NOT ride in this groove during cycling, and may or may not be found fully seated there during disassembly. This has been true of all Glock designs, moving from the original G17 to the Gen3s, and I assume the latest Gen4 continues it (I have very little experience with the Gen4 guns).
I can't really speak to the strange re-assembly problem (not being there), but if I was asked to guess, I'd say that the slide was re-mounted with the recoil spring NOT seated in the cutout (perhaps it was against the second groove/lug on the barrel), and it got wedged/bent when the slide was cycled.
"Placement is power" -- seen in an article by Stephen A. Camp
(RIP, Mr. Camp; you will be remembered, and missed)