Compiler Warning Messages

Expressions associated with portability problems
Implicit type casts and conversions between different but compatible types
The use of a constant in place of a conditional expression in if or while statements
Functions that do not return a value although they should
Variable declarations that mask other variables with the same name
Missing enumeration members or extraneous values in a switch statement
Unknown #pragma options
Unreferenced variables, structure members, or labels
Failing to initialize a const declared object

(Depending on the language, some of the above may be errors)