There is one specific plane type (Airbus A320) that has significantly more fume events than all other plane types, suggesting that plane type or associated engine has a design or procedural flaw.

Since airlines consolidate on specific types, it is possible that some travelers experience this problem often and others very rarely.

Through no choice of my own, I seem to only fly on Boeing 737s, but the one recent time I was on an Airbus (A321neo) there was a distinct exhaust odor upon engine startup.

In my experience, a large fraction of flights have very stinky air during a substantial portion of the time from when the engines start until takeoff. Whether this is “gym sock” or something else has never been entirely clear to me.

I’ve never noticed this in 35 years and I have an unusually sensitive sense of smell.

Of the last 50 flights I took, none had gym sock smell.

If anything smelled it's usually due to a passenger. But nothing from the aircraft itself