Take the 2-minute tour ×  In a murder mystery, most of the story
is generally focused on figuring out who the murderer is; in
"Fair-Play" mysteries, it's assumed the murderer is a
significant character in the book, and the fun is in figuring
out which of them it is.  This being the case, readers familiar
with the format generally try to second-guess the investigation.
That is, not only do they try to solve the case along with the
sleuth and the unfolding information within the story (which is
fine, that's something I want); their guesses are often based
upon their knowledge of the structure and conventions of mystery
stories - along the lines of "Mr. X can't be the murderer,
because he's the first guy the detective suspects," or "It's got
to be Little Miss Innocuous, because she's being portrayed as
being innocent and trustworthy - so she's the one the author
doesn't want us to suspect!".  Now, plenty of mysteries can
withstand this sort of meta-interrogation, and some
intentionally subvert these conventions.