Yes, this has happened to me twice.

Once was with a cel shop that is no longer
open several years ago.  Got confirmation that
I got a Nuriko cel then got email couple
hours later saying someone else got it
first.  Kinda unbelievable that I was first,
then wasn't - I suspect they got a large offer
of money and decided to make up a flimsy excuse.

Other was a private collector selling cels.
Had a fox Kurama that I got confirmation
that I had, even spoke to person selling on
phone and they said I had.  Then get email
day later saying that they had cancelled me as
female foreign collector had offered to buy all
their cels for sale as long as she could get the
one that was already on hold for me.  I was so 
pissed.