mistaken qualified infix syntax could have a nicer error message
Consider the expression
Not in scope: data constructor Prelude'`.
Unfortunately this is valid code if the module-referencing name also is the name of an in-scope data constructor. In the common case that it isn't, at least (a cursory inspection doesn't find any such exports in the libraries, e.g. Data.Map doesn't export a data-constructor Map), a message to the effect that infix operators must be qualified like (Prelude.+) instead, would be nice. (hierarchical module names probably make this a little more complicated somehow)