I agree with Turbo...never. Something's not right. I have noticed old "N-numbers" appearing. This is when the number has been re-assigned and the "new" aircraft has not yet flown in the "system." When I checked the date of the "old" flight, it was sometimes as old as 11 years.
Someone on this forum flies an RV-7, but apparently in September of 2000, he was flying a PC-12 with the same N-number.
P.S In light of Randy's and John's posts below, I see how it could happen. So even though I have never had their experiences, I see how it could happen.