What is the problem?
The problem is that the ejected casing will fly straight back to the shooter's forehead or to top of head instead of to the side. This could result in casing falling in to back of the shirt or hitting the shooter in the face.
Here's a video from TWANGnBANG who got hit with the problem when he started making video to disprove the problem.
Here is the link to part 2 of his video
How frequent is the problem?
According to a Glock rep, this occurs to about 1 out of 100(1%). However, my conjecture is that this percentage is lower than it actually is for a few reasons.
First, not a lot of people shoot many rounds. Ejection problem tends to appear after 1000 rounds or so.(Some report just after 300 - 500 rounds). If someone just bought a Glock and not shoot much though it they wouldn't reach the point where the problem surfaces. Second, not everyone talks about their problem on the internet. There are plenty of opinions that get amplified on the internet, but there are just as many people who do not post their concern on the internet.
Glock has not made any official statement about this problem and treat is as a sporadic problem.
What is the cause?
There is no definitive answer to this question. Originally, it was attributed to stronger recoil spring used in Gen4 Glocks. The first ejection problems were reported with Gen4 Glock 19s to the best of my knowledge. I've witnessed a Gen4 G19 have ejection problems and the well known instructor who was running the evaluation was not happy with it. Eventually recoil springs were updated. At the same time, Gen 3 Glocks started to show same problems.
With this new development, people focussed on ejector and extractor. Since both parts were used in both Gen3 and Gen4, it was believed that they are the most likely culprit. A lot of attention was given to the dipped extractor(ones with Loaded Chamber Indicator - LCI) which was introduced a few years earlier. Many people posted photos of their extractors and looked for some difference in the previous dipped extractor and ones causing the problem. It seems like there is some difference in different batches of extractors.
Ejectors were also under scrutiny and Glock responded by changing the ejector for 9mm versions. The ones causing problem were marked 336 and the new version is marked 30274. Newer ejectors have more surface area to strike back of casing. However, it is worth noting that ejectors marked 336 were also present before the ejection problems started. So not all 336 ejectors are bad.
336 ejector on top, 30274 ejector(new) at the bottom
How can I fix it?
This the part where we have to find solutions by trial-and-error.
The easiest way is to send the gun back to Glock and they will work on the problem. From what I heard, they work on your gun and see which would solve the problem. However, I've also read that for some, the problem comes back after awhile after some use. There seems to be a number of people who did not have problems after trip to Glock factory, so it may work.
Or if you are a GSSF member(Glock Sport Shooting Foundation) then there will be a Glock armorer providing services for your Glock at an official match. You can take your Glock to the match, shoot and enjoy, then take it to armorer to get the problem fixed.
If you are sure that it's uniquely an ejector problem, then you can take a file and use it once on the surface where it makes the contact with casing. This will increase contact time between the rear of casing and the ejector to give it a better ejection.
For those of you who can detail strip a Glock correctly, another way is to work on it by yourself using aftermarket parts. Glock does not ship it's factory part to end users, but only to distributors and armorers. Thus as the owners of Glock handguns we are limited to aftermarket parts.
You can try after market extractors from Lone Wolf or APEX Tactical.(Gen 3, Gen4)
At the time of this article, APEX is still working on refining their extractor to work for both Gen 3 and Gen4 so it might take some time for ejector to show up for public.
Recently I came across a post from SIGForum's caneau who fixed his G19 problems by doing these.
--Replace the extractor with a 15-degee, non-LCI(older model, non-dipped extractor). It has to be non-scalloped version and replace spring loaded bearing as well.
-- Get Lone Wolf Glock 19 guide rods, Gen4 Adapter, and 15 or 16 lbs ISMI spring, and replace the recoil spring assembly.
He also got grip tape to ensure he grips the gun firmly.
Conclusion
At this time, there is no definitive answer on how to deal with this ejection problem. The cause itself is unknown and it has been left to end-users to figure out, which resulted in some briliant minds working on the problem and slowly fixing it. Glock has not made any official statement about the issue and seems like it will be dealt as a case-by-case basis instead of issuing a notice for fix.
There is a long thread on M4Carbine.net that chronicled this struggle. Also you can check Glock Talk's thread on the matter.
For those who wants to stay up-to-date on this issue, these would be great places to read up on it.
