Anyone ever get someone elses mail but don't notice that till you open it?

I've had that happen a couple of times, I think its mine cause it was with my mail only to find out its not. a couple of days ago I got this letter and I was wondering why housing was writing me, I opened it and saw it was a form to fill out and it had this guys name and address on it, they never wrote the address on the envelope just the name of my place and no ones name for the address, it was someone from the apt bldg that they just made a yr ago, behind my bldg, anyways, I went there and knocked on his door, told him I didn't mean to open it but I thought it was my mail at first and I saw his name and address on the form so I just wanted to give it to him, he just thanked me, another time I was opening my mail and this check comes out and I'm wondering who's sending me a check, I looked at it and it was from a lady that was in rehab writing a rent check and mailing it, the ink on the envelope was so dry I couldn't read it, I gave it to my apt manager, he just thanked me for it, glad I noticed it before I signed it. another time, I got my neighbor's mail, shes lucky I got it cause I opened it by mistake and it had all her credit card info, shes lucky I got it cause I'm honest enough not to copy it, I gave it to her and told her I opened it by mistake, yrs ago I got an office check for the manager in my mailbox, she wasn't there that day so I left it under her door and said sorry I opened it, thought it was mine till I saw that. glad I noticed it before I signed it, she never said anything, if I get someone's mail in my bldg I just give it to them cause then they'd have to wait a week while it travelled if I put it in the box, if its mail from outside the bldg they gave to me then I just put it in the mailbox. I feel bad for opening them but sometimes I think its mine cause it was in my box and don't notice it till I open it, I know you're not supposed to open other people's mail but I heard if its an accident that's a different story, anyone ever have that happen where you get someone's mail and don't notice it till later on.