What cars didn't meet homologation numbers?

The Ferrari 250 GTO is the classic example. FIA regs denoted 100 has to built, but Ferrari only built 39 (or 33, depending which you define as true 250’s).

They got away with by numbering chassis out of sequence, making it look like 100 existed without need of proof. But then inspectors visited the factory to se the full 100 cars, which Enzo got around by putting a certain number of cars in a room at any one time.
Enzo promised that the rest of cars were in other rooms, but what was actually happening was the Ferrari boys were shuffling the same cars into each consecutive room as the inspectors visited them. So the inspectors might have seen 100 between 4 different rooms, where instead they only saw the same 25 each time.

Lancia pulled a pretty good one.

Back in its day, the rally rules required at least 400 examples be built. So naturally, the inspectors are are gonna noticed if there’s only 200. So what Lancia did was say that that lot of 200 was full, and that the other 200 were in a lot on the other side of town.

Here’s where the ingenuity comes into play.

On the way to the other lot, they stopped by to get some lunch, dragging out the meal for as long as they could. Meanwhile, the original 200 Lancia’s were being moved to the other lot. Once lunch was over, they moved on to the other lot, where the other 200 cars where.

The Grand Tour actually did a segment on it.

Wow, i hated that type of BMW. But if i look on this pic it looks so cool man…

The E46 M3 GTR is a good example. BMW wanted to beat Porsche in a racing series but they had to homologate it. Something like 10 cars originally (to make Porsche happy) and was later upped to 100 cars and 1000 engines. But, because the V8 was so expensive to make, BMW dipped the series before they were required to build them. So there are supposed to be around 100 M3 GTRs on earth and 1000 V8s that were in them, but they only made about 10