* ending of the last movement of the Jupiter Symphony
* Introitus of the Requiem (four-voice ricercare in the instruments, then cascading upward in SATB choral writing, both combined with an operatic-style accompaniment)
Here's a mind-blowing one-- at the end of his String Quartet K. 387 in G major. This is a modified rondo-sonata form where the rising figure of the rondo theme-- normally just a simple, catchy, tune-- is actually the subject and answer of a fugue! I'm fairly certain Mozart is the only composer of his era to substitute a fugal exposition for the rondo theme in a modified rondo-sonata form[1].
Even more impressive, the 2nd theme is a different fugue which continues into the rondo theme for-- you guessed it-- a double fugue.
All of this is so fast, effervescent and seamless that someone like OP could be forgiven for not even noticing the fugue. So now OP must ask-- how many other examples are there of Mozart subtly shifting into and out of first-rate Bach-like contrapuntal textures?
It's a question Beethoven undoubtedly knew the answer to, at least for all the Mozart scores he could get his hands on.
1: Haydn has a great musical joke at the end of the so-called "Lark" quartet-- in the middle of a fast, old-school Rondo he switches to minor and plonks down a thick four voice fugal exposition and development against the fast-flowing figuration from earlier. It disappears as quickly as it came, like Carl Stalling but before the era of cartoons!