1st I'd like to say the warning contained in your post should be "Warning to anyone who performs any repairs or maintenance on their own vehicle: ensure every single tool you pull out to complete the repair/maintenance is accounted for when the job is done as to not leave or even lose anything in your engine bay". 2nd This is another great reason to have your toolbox (or whatever you store your tools in) organized so it's easy to see if you're missing anything at just a glance. Sockets are, imo, the easiest to lose tool in the arsenal. Buy yourself a socket rack and get just enough socket clips for each of your sockets. Store the unused extras somewhere (not on the rail) so you don't confuse yourself and you can quickly identify how many sockets are missing and roughly what size they are. Hope this helps for the future. As for the nut loosening, sounds like it could've been something that just wasn't torqued properly when it was made. That's more of a manufacturer defect than anything. The lesson I take away from this is always double check torque on parts yourself when you weren't the one to witness the torquing. Only way to get something done right is to do it yourself, right?