They're real at least. Suggested by Jason Shih who has seen too many to count. Here is the first of two photos of a Paleocarcharodon orientalis shark tooth. This is a relatively rare species, and fine, complete specimens are in demand from collectors. I bought this tooth specifically to demonstrate one sort of forgery.
Imperfect teeth lacking the cusplets are considerably less valuable.
So this tooth has the tips of two teeth from another species glued in place. The serrations were ground into the edges with a file or jeweler's saw, and the dirty work was hidden with matrix mixed with glue or resin. Note also the telltale break in the tooth margin just before the cusplet. This one is obvious to the experienced collector, but the unwary can be taken. Suggested by Gene Hartstein who says I could fill a whole page with these.