Well, as a lab tech, I ran hcg tests on PLENTY of women whose husbands had had vasectomies...and SURPRISE!!! Almost 90% of those + tests were men who didn't go back for their SECOND follow up. The farthest out from a vasectomy that I saw come out positive was 13 years. Can you imagine?!? After 13 years, one would assume the vasectomy took! But nope! In one case in particular, there was a lot of rumor/mud slinging about affairs, ect (of which I knew the woman and knew almost for certain there was no affair) and when Mr. went in for a LOOOOONG overdue 2nd post vas, there were clear swimmers!
Is it common? No. But I've seen it plenty!