Parts breakage?
The result of normal wear & tear?
An unexpected premature failure which may be caused by an unknowingly defective part?
Something caused, even if unintentionally, by the owner/user?
Something which came as a stock part from the manufacturer, or an aftermarket part?
You use handguns long enough you're probably going to wear something out or have something break.
Kind of like with other mechanical devices like cars, trucks, motorcycles, boats, etc..
As a LE armorer for a number of different types of firearms I've seen my fair share of parts wear, breakage and damage.
Limiting the examples to just the 99 series pistol mentioned by the OP, as a S&W trained P99/SW99 LE armorer I've had to replace the occasional damaged, broken or worn part. As far as just the ejector goes, though, I've seen a few of them break over time.
One I remember involved a .40 S&W pistol in which a 9mm ejector (part of the 9mm sear housing block) had somehow been installed in the .40 S&W gun.
However it happened, it had somehow withstood many thousands of rounds, according the owner, as a loaner gun in LE he classes he taught.
Had another one break off in less than what was probably 2,000 rounds ... and another one break off after having been used in a .40 S&W SW99 which had been fired for upwards of 50,000+ rounds. That last one seemed pretty darn reasonable, to me. Well earned.
I've been told of a SW99 kept at the S&W Academy which has been used for loaner/demo use and has reportedly fired more than 75,000 rounds, without repair (or even cleaning, I was told) and without malfunction. This falls within the category of "Don't try this at home boys & girls".
Why do you think that firearms companies which maintain LE armorer training programs recommend that dedicated LE guns should be inspected periodically? Things wear out ... things break ... sometimes even a new part may break prematurely because of an unknown defect in manufacturing ... and sometimes the users contribute to the process in some manner.
:banghead:
Same reason manufacturers have warranty repair services, you know.
That's why they make spare parts, too.
Things can happen.
I liked how an instructor politely phrased things in a Glock recert class I attended ...
He said to replace recoil springs every 2,000 - 3,000 rounds ... and if we're seeing broken locking blocks, locking block pins and trigger pins then we're not replacing the recoil springs soon enough and the guns are being unnecessarily battered.
We were also essentially reminded that other parts are not really 'eternally lasting', as well, such as slide stop lever springs and trigger springs. Magazine springs. Slide stop levers (springs). Followers. Magazine bodies. Trigger bars. Firing pins. Firing pin springs. Firing pin safety plungers.
You get the point ...
Machinery wears out and breaks. It may understandably last longer with proper maintenance. This may mean replacement parts peridocially.
Hey, I've known some folks who could break a ball bearing.