I don't know if this happens only to me... so I would write here to know if I'm the only one, or there are some other "lucky" ones like me! 
Example 1: DVD movie; I was sure I had that title... asked to others in the family, someone remembered it, someone told me I was crazy... I searched a lot between the titles scattered around different rooms in the house - they are few hundreds, after all... nothing... searched in unthinkable places... nothing... so I thoght that maybe I was crazy... few days later, I visited my sister, and she came out with "Do you remember I borrowed THAT movie from you some weeks ago? No? Well, maybe I picked it up without asking you..."
lesson number one: write down every title you lend and borrow, and ask friends and relatives to inform you about them...
Example 2: LD movie: after the last quest, I thought "may be this title at someone's else house? not a laserdisc, for sure!" and "are you sure you have REALLY this title? yes, according to the list I made that is 100% accurate", and also "why don't you put them in SOME order? because I'm lazy and... hey brain, shut up!"
So, I began from the shelf #1, trying to find it just reading the titles of the movies on the spine with the right colors... many shelves after, nothing... maybe the spine color wasn't right... restart, fast reading... an hour later, nothing... OK, probably I missed it, time to read the title one by one, not so fast... well, I found it... at the end... it was near the 3000 point out of 3200... lucky as usual!
lesson number two: put your movies in "some" order; possibly, write a list in a spreadsheet that points each title to a shelf position - it will be easier to spot it between fifty ones instead thousands!
Example 1: DVD movie; I was sure I had that title... asked to others in the family, someone remembered it, someone told me I was crazy... I searched a lot between the titles scattered around different rooms in the house - they are few hundreds, after all... nothing... searched in unthinkable places... nothing... so I thoght that maybe I was crazy... few days later, I visited my sister, and she came out with "Do you remember I borrowed THAT movie from you some weeks ago? No? Well, maybe I picked it up without asking you..."
lesson number one: write down every title you lend and borrow, and ask friends and relatives to inform you about them...Example 2: LD movie: after the last quest, I thought "may be this title at someone's else house? not a laserdisc, for sure!" and "are you sure you have REALLY this title? yes, according to the list I made that is 100% accurate", and also "why don't you put them in SOME order? because I'm lazy and... hey brain, shut up!"
So, I began from the shelf #1, trying to find it just reading the titles of the movies on the spine with the right colors... many shelves after, nothing... maybe the spine color wasn't right... restart, fast reading... an hour later, nothing... OK, probably I missed it, time to read the title one by one, not so fast... well, I found it... at the end... it was near the 3000 point out of 3200... lucky as usual!
lesson number two: put your movies in "some" order; possibly, write a list in a spreadsheet that points each title to a shelf position - it will be easier to spot it between fifty ones instead thousands!