Gantry assembly randomly fell out help.

[Amer11235]

Last night while my printer was NOT in use, the front right y axis gantry assembly fell out. I managed to reassemble it and put it back in place and the printer seems to be working, but I found a spacer that I couldn't figure out where it belongs. As far as I can tell, I reassembled the gantry just like the others. You can see in the pictures there's the wide spacer to the inside(left of the rod) and the thin one on the outside(right of the rod) . Here is a picture of the mystery spacer, It's thinner than the ones I saw on the other gantries and it's clean, which leads me to think it might be a spare part that got mixed in while I was putting the gantry back together. Any Idea where this spacer belongs, did I miss something or is it just a spare? Also, Has anyone else had a gantry randomly fall out, is this something I need to fix? I appreciate any insight you guys can provide. Thanks!