JonathanM

Sure - it was under the motor, not the batt box. In the sub-construction that begins at step 165. At step 166 you want two 1/2 pins 4274 in the 32291 so they connect to the underside of the end of the M motor. Interestingly, the instructions on the web don't seem to show them, so perhaps they're no longer used, but I could have sworn they were shown in the image of the complete sub-construction at step 165 on my copy (purchased ~1 month ago). They likely aren't really required - just gives a bit more support to the back of the motor. (Apologies for somewhat off-topic)

I had this happen last night with the CLAAS 42054, though not after completing it, rather after packing up for the evening I noticed I'd missed a 5M beam. Fortunately it was easy to see where it was missed as it was only 5-6 steps back. But it was quite a bit of disassembly to get it in place as it had to go in just before pushing some axles into place which were tricky to get back out again :) As to mistakes, I've found the last 2 sets have had errors: The CLAAS misses the two 1L pins under the battery box at the front, while 42066 has the 3L axles with stop mislabelled as 4L with stop axles throughout in the main images (it's correct in the parts lists). I guess it's understandable with these big sets for a few mistakes to creep in, even with all the QC they go through.