how would someone make a mistake like that? it doesn't seem probable.
I guess it's always possible to use something like this for money laundering.
I.e.: (1) Send 1BTC to address whose private key is publicly known
(2) Sweep the funds a few minutes later (depending on the amount of
plausible deniability vs.
risk you're willing to take)
(3) Claim you mistakenly sent the funds and that they're gone
Then mix and be happy. Just an idea.