I guess it depends.
Firstly, there are specific areas/regions on the head and body that are vulnerable. If you strike these areas at the correct angle and with sufficient force you will cause an involuntary reaction. Examples include the solar plexus, kidneys, liver, the temple and the chin. If you hit the chin at the right angle it will pinch a nerve and the opponent will be knocked out immediately.
So with regards to this first point anybody can get knocked out or stopped if they are struck in these areas/regions. Therefore getting "caught" would mean didn't properly defend themselves or read their opponents attack and were struck in an area that invariably leads to that involuntary response. This interpretation is merely someone respecting that they are mortal and in a fight they can get "caught"
The second interpretation is when a fighter doesn't recognize or respect the technical side of fighting. They claim they got "caught" but they don't take responsibility for the fact that their recklessness or poor techniques are what created the openings for them to get "caught". In this interpretation the fighter is potentially making the claim that the other fighter won on luck and not on strategy and execution.
For example look at both Werdum vs Miocic and Aldo vs McGregor.
In Werdum vs Miocic, Werdum chased after Micioc without respecting boxing range, he didn't use ring generalship to cut Miocic off and he chased Miocic by switching his stance and walking forwards rather than maintaining a boxing or muay thai stance. He didn't keep his chin tucked or maintain any defense. He used poor techniques and was reckless and got the exact result someone should expect from fighting that way. There was nothing lucky about the result for Miocic other than the fact that Werdum was foolish enough to fight that way.
Similarly Aldo rushed in at McGregor and also didn't keep his chin tucked. He lunged in with a hook when we was out of range and that left him exposed to eat a cross. McGregor beat him because of a technical flaw from Aldo and his recklessness.
Any fighter can get knocked out so getting "caught" is normal. But it should also educate a fighter that they left themselves exposed defensively or that their offensive tactics were used in the wrong context.