I bought Led Zeppelin IV but the record inside, including the inner sleeve, were from Genesis - Trick Of The Tail. I can see how the mistake was made at the pressing plant;. The catalogue number of IV was 19129, and the catalogue number for ATOTT was 36129. Somebody got three of the numbers right. I don't know how many boxes were shipped this way. Eventually somebody had to notice, right?

Yes, and at that time, Columbia in Don Mills was doing contract pressings for Atlantic, Atco and related labels. So the mixup occurred in the packaging department. I was working at Sam The Record Man's main store on Yonge Street in Toronto (the largest record store in Canada) then. After opening my album to find the wrong record, we took all the most recent copies out of the bins upstairs and waited for replacements with the right record.

Also, they were both from Atlantic!

I bought a first pressing of PJ Harvey's Dry, with the bonus demos LP, and when I got home found that it held 2 copies of the proper LP but no demos LP.
However, the sleeve had no track listing for the demos LP and although the labels were identical I just assumed that was the way it was. Even when I played both LPs my initial thought was 'waste of money, there isn't any noticeable difference'.
Some months later I was back in the record shop and commented that there wasn't any difference. They did actually have a spare copy in the back so they bust open the seal and to everybody's surprise there was 2 pressings of the demos LP inside. Needless to say I got to do a swap after returning my copy, but it was pretty amazing that the one they pulled out from the storeroom happened to be the one that they needed to see.
Note that vinyl LPs sold in tiny quantities back in those days and I was probably one of the few awkward customers that asked for them.

I would say that the most likely reason for the confusion is that Led Zepp IV has no writing on the cover whatsoever, so perhaps quite easy to make that mistake.

There is a new double compilation "Best Of Teddy Pendergrass" put out by Philadelphia International (Sony), pressing origin unknown but it is sold in the UK. Unfortunately a large quantity of them come with sides 3&4 of a similar O'Jays compilation. I don't know if Teddy's lost sides are to by found in the jacket of the O'Jays comp.

I bought a cheap two-disk Steely Dan comp a few years ago from Sainsbury's. When I played disc one, I quickly saw that it was playing disc two's tracks! I thought "Oh no, I've got 2 x disk two and I'm going to have to go back and get it changed!" Anyway, I then played disc two and it had disk one's tracks!

Result!

There must've been a lot of them like that! If someone forgot to change the labels in the press when they switched plates, They probably did a number of them before it was either discovered or the correct labels started showing up. Maybe they only had a few of the wrong ones in and put the right ones in on top of them and just let the old ones run out.

Sorry, I neglected to mention these were CDs...