TankieJerk

Can't remember which aircraft it was, but there's a story about how one of their copied models had a mysterious hole drilled in it, which was missing from the originals. A good amount of time was spent speculating on what its purpose was. Was it an improvement of some kind? Some new hardware? A space for a specialized camera?

Nope, turns out the model they copied from had a extra hole drilled there by accident, and not knowing what it was, the soviets just kept it.

Then they took the planes apart and copied them as closely as possible.

Which was tricky given imperial vs. metric supply chains/production capabilities:

The Soviet Union used the metric system and so sheet aluminium in thicknesses matching the B-29's U.S. customary measurements was unavailable. The corresponding metric-gauge metal was of different thicknesses. Alloys and other materials new to the Soviet Union had to be brought into production. Extensive re-engineering had to take place to compensate for the differences, and Soviet official strength margins had to be decreased to avoid further redesign.[11] Despite those challenges, the prototype Tu-4 weighed only 340 kg (750 lb) more than the B-29, a difference of less than 1%.[12]

https://en.wikipedia.org/wiki/Tupolev_Tu-4

Wait what, Russian government is selfish and acts like an asshole?

Who would've guessed.

Oh right, my ancestors who fell fighting them.