What Causes An Instrument's Neck to Warp ?
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When wood is subjected to extreme temperatures and humidity it becomes vulnerable to warping. A prime example of this comes from the construction of a cane chair where the wood is put ' into a pressurized steamer until it becomes flexible enough to be easily formed into it's shape. This is essentially what is happening when the neck of an instrument is subjected to high temperatures, such as an instrument that is left in a hot gig trailer or trunk on the way to or from a show. Once this has occured, the neck will remain in that position even once the string tension has been relieved. Our process of heat treatment will allow the neck to return to it's original state with just the right amount of relief for proper setup
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http://www.warpedneck.com/what_is_a_warped_neck.html

Hmmm ...
no mention of frets!

"Just me two-pence mate"

Now that you mention it, I recall I once had a
Gibson Les Paul bass that I was able to buy quite
cheaply because it had a pretzel neck. A great
luthier in Rockford, Illinois straightened it right
out with a bending iron or something. Left some
scorch marks on the back of the neck, but it played
very nicely. Still sounded like a bucket of mud,
but hey, that was what I was into in those days.

J