I have a few of them.  None of these should be 
taken too seriously.

- A dealer (or an eBay seller) advertises cels 
from a favorite series, but when you actually 
look at the cels, you find they're from an 
uninspired sequel/remake rather than the 
original (i.e., "Bubblegum Crisis cels" 
translates to "BGC 2040 cels" and "Macross" 
means "Macross 2").

- A dealer updates their site (triggering Keep-
Track), but when you go there, you find the 
update was only to publicize eBay auctions.

- A seller advertises a "NUDE" cel but the cel 
pictures a character from the neck up.  ^_^;

- A seller lists the same cel repeatedly until 
it's been so "overexposed" that no one wants it, 
even if it is a great cel.  (I think one thing 
people like about collecting cels is owning 
something that's unique.  If lots of other 
collectors have seen a cel and passed on it, it 
can make you less enthused about owning it.)