I don't want to say "it'll never happen", but, right now, I think the likelihood of it happening is small since I never leave the car *w/o* DD (she doesn't go to daycare or a sitter or anything like that). So, it is very strange to NOT have a baby in the car w/me. But I can see how if the OPPOSITE is true (baby usually isn't in car, or in that car, or whatever) it could slip your mind.
That said, I did once drive home only to find that I hadn't buckled the bottom half of DD's harness...only the top part was done. Thinking back, she had thrown something onto the other side of the car and was screaming for it, and I went to get it for her, and just completely forgot to finish the snapping in...now I make it a point to double check the harness...so, obviously, I'm not infallible.
PS...both my husband and I were "forgotten" as children (not babies). My parents momentarily forgot me once in a store...but were back pretty much before they left the parking lot. My husband actually got left for a while...they'd driven all the way home before they realized they'd left him behind...