This is a trivial mistake in the sense that -Wall gives two
warnings about it:
Warning: Defined but not used: `s'
Warning: This binding for `s' shadows the existing binding
bound at Control/Monad/State/Class.hs:62:7
However, it caused a very hard-to-track bug in a completely
unrelated library (fb, which depends on http-conduit, which
depends on tls, which depends on mtl).