The magic thing is "generated interface checking". When the compiler sees a routine that is not a module or contained procedure, it generates a specially-named module containing an interface block for that routine. In version 10.x, the module is routinename_MOD and the file names are that name plus .f90 and .mod. (This will change in a future release to reduce the chance of a conflict with a user module.)
When the compiler sees a call to a routine for which no explicit interface is available, it looks to see if there is a generated module for it, and if so, uses the interface block to perform error checking. (Note that a generated interface is NOT a substitute for providing an explicit interface when the language requires one.)
The compiler tries to be smart about the case when the called routine is later in the same source in that if it does not find a generated module, it waits until the whole source has been processed before doing the checking. The problem occurs when you compile the source, edit it to change the called routine, say, removing an argument, and then recompile. This time the compiler sees that a generated interface is available, not noticing that it would be recreated later, so it uses a stale definition.
This is controlled by the options /gen-interface and /warn:interface. Both are on by default in newly-created VS projects.
