These machines are crafted using the highest level of precision in the industry. It sounds as though one of the three locking points isn't being properly aligned when you close it, or one of them has a faulty construction--and I'd bet on the first being the real problem. When you close the bottom cover by rotating it into place, the three pins and sockets on the inside of the cover all lock into place. It sounds like one of these has not been seated properly when closing the cover into locked position. Check carefully that you are closing it properly, and take advantage of your free AppleCare telephone support so they can explain and verify if there is a mechanism problem. But this is NOT common nor acceptable, because like I said, all the pieces of this machine have been precision designed and manufactured to have absolutely no gaps and perfect fit and finish.

@Steve Booth

I am in the exact same situation now.

My board assembly was snugly stuck (most are) and using only enough force to budge it at all also caused it to slip far enough as to crack apart my ZIF connector shells. ZIFs are not so difficult so I have NO idea why the author wouldn't recommend removing then all for safety. Or noting--INLINE WITH THE INSTRUCTION--that they all be disconnected before sliding the board assembly though not entirely necessary.

If you know where I can find the temperature sensors, please let me know? I really don't want to have to take this back to the Genius Bar and they see what I've done. It's humiliating. I should have known better. And had I just been exploring on my own instead of mindlessly following this guide step-by-step, I would have removed all my cables first before trying to budge the logic board.

Very disappointing.

Ifixit note, Mac mini mid-2010 step 16

@tdowse1 I agree completely. I take issue with this guide because I believe it has an error. With just one of two changes I wouldn't have destroyed my ZIF sensor connectors:

1) Issue the warning BEFORE or even with the instruction. Following the instructions step by step and not reading a step ahead, you don't realize you can damage your cable irretrievably. Even if you read the entire guide first, you can't be expected to memorize all warnings BEFORE you act on the instructions.

2) Instruct the user to disconnect ALL of the ZIF cables first. This is truly the only safe thing to do.