I dunno if I should admit to this
I once landed after an hour or so of local flying and realised I'd not strapped in!
So much for pre-flight, and pre-landing checks
Many times when I found out at the hold, pre take off. Within the last 3 months I got it up to 200' before I noticed. Reason being - no holding point, power checks had been completed on the RW.
Would never admit to having gone as far as you though, especially in a 737 in commercial ops.
(OK, harness was on, just hadn't rotated the buckle release knob far enough back to secure the shoulder straps (I thought). Got downline and found out it also hadn't fully secured the lap strap either (while wriggling to look out for GSU equipment).
RIX