One issue is confirmed to cause this, the newest extractors are much more likely to be out-of-spec and not move freely enough in the slide, or for whatever other reason not hold the rim of the spent casing firmly to the breech face.
The other issue is speculative, but I believe this to be one of the causes. My Gen3 G27 had the ejection problem, my Gen3 G23 ejects fine. I took the extractor from my G23 and put it into the G27 slide. In comparing how the extractor sat in both slides, I could see that the extractor sat further forward in the G27 slide. I could also see that the forward portion of the extractor protruded slightly from the side of the slide. With the rim of a casing under the extractor claw, there was noticeably more of a gap between the claw and the breech face for the rim to slide forward and rearward between in the G27 slide. Also the casing was held with noticeably less tension in the G27 slide. This leads me to believe that Glock has a quality control/manufacturing tolerance issue in the machining of their slides, I believe in some slides the cut-out for the extractor is positioned incorrectly.
My Gen3 G27 ejected fine for the first 200 rounds, then some of the spent casings started going over my head. By 350 rounds some casings were ejecting straight up in the air. I also noticed a lot of brass dust and shavings collecting on the ejection port, with corresponding triangular dents/scrapes on the case mouths of 98% of the spent casings. I installed a new extractor but it didn't make much difference, the case mouths still had scrapes and some casings were still going over my head, but it did eliminate the straight-up ejections. I then installed the new .40 ejector stamped 28926 and it fixed the ejection issue completely, there are no more dents/scrapes at the case mouths of the spent casings and all now eject to the right between 3 o'clock and 5 o'clock.
The new ejectors were originally used only in Gen4. When Glock's Warranty Department saw the same symptoms in recent production Gen3 guns, they eventually decided to start putting the new ejectors into Gen3's to fix the problem.
All 9/40/357 Gen4's with a test-fire date after 10/10/2011 will have the new ejector already installed. Gen3's still have the old ejectors, but if you have ejection problems with a Gen3, Glock will install the new ejector into it to fix the problem.
If your gun has the ejection problem, here's what you should do:
1. Get the new ejector, 30274 for 9mm or 28926 for .40/.357
2. If the gun still ejects erratically with the new ejector, that means the extractor needs to be replaced
To get the new ejector you have two options. You can find a local Glock Armorer to order and install the new ejector for you, or you can call Glock and arrange with them to send the gun in and have it installed. When you call Glock, talk to the Warranty Service Department and explain the problem to them, tell them you'd like to have the new ejector installed. They'll tell you to find a local Glock Armorer. If you don't have one near you who will do it for you, tell them that and ask that they send you a prepaid shipping label so you can send the gun to them and have them install it.
Apparently one of the representatives in the Warranty Department is quite unfriendly and will refuse to send you a prepaid shipping label, he may even say that they don't install the new ejectors into Gen3's. If you get him on the phone, don't get discouraged, just call back another time and hope that you get someone willing to help you.
If you have a Glock with the old ejector, 336 for 9mm or 1882 for .40/.357, it does not need to be upgraded unless you experience ejection problems.
Remember seeing it also. My g30 will throw one at me once every 3-4 shells
There is no new ejector for .45. I don't know if the newest .45 extractors are different from the old ones, but if they are I'd imagine that's the problem. The only thing I could think of for a G30 is to try a new extractor, possibly a LoneWolf extractor:
LoneWolf brand extractors are made pretty well, they're a clean and crisp form of investment casting just like Glock's good old extractors were.
I haven't yet heard from anyone who's tried a LoneWolf extractor for .45. Several people who tried the .40 version had good success with it.