I haven't seen it with BPs yet (only been into them for about two years now) but I have had it happen with blood pythons. I bought a small adult WC "male" in mid 2005. In late 2006, after "he" had bulked up quite a bit, I tried pairing "him" with a couple different known females, but they didn't mate. One day I looked in on them and found the WC "male" laying a clutch of eggs.

Unfortunately I didn't have an incubator set up yet (I'd only been pairing them for about a month, so I figured there was no way I could have eggs ..!), and the one I slapped together didn't quite do the job, so I only got a baby to within about a couple weeks to term. It was a fully-formed baby, though.