With all this talk of modern art fakery (which, as you say, is so rife that I've already experienced an extraordinary amount of it during my relatively few years in the trade), I thought I'd share you my favourite personal experience.  In my naive collecting days (not that long ago...) I purchased for a modest sum of around £100 a watercolour on Ebay purporting to be by the hand of Kyffin Williams - the main reason for this acquisition was the fact that I found the work had been offered as a genuine some years previous by Bonhams.

It had failed to sell but I thought the estimate was a touch high and thus I approached them to see if they'd consider re-offering the piece at a more conservative price.  They confirmed it was the same work but said that they'd have to get an 'external specialist' on the artist to inspect it first hand to reconfirm the attribution.  I felt this was probably a bad sign and thus was amazed when they came back and said the expert had proclaimed it to be right. Hence they re-catalogued it in one of their forthcoming sales [now withdrawn].

However, the catalogue had not been live long when I received an e-mail from Bonhams informing me that it had been brought to their attention that the work was a fake as its source had been revealed as former genuine sketch by a completely different artist, the notable but less expensive Alan Lowndes [below].

Someone had cut of Lowndes' signature, added a 'KW', some Williamsesque splodges, and crosses on the spires of a North of England pavilion to make it look like Venice!!   

Quite how this fooled Bonhams, twice, and an apparent expert of the artist's work is beyond me but I guess the story acutely demonstrates much that's wrong with the murky world of modern art - and why I now almost exclusively stick to old masters/pre-1900 pictures which thankfully are infinitely more interesting both academically and aesthetically...