Are pilots expected to keep an ear out for conflicting authorizations?
The short answer is yes, they are definitely expected to listen and act accordingly. This is one example of situational awareness in aviation, and pilots are expected to use all useful information sources to maintain awareness of what's going on around them.

A nice example is this one where the crew of a US Airways aircraft avoided an accident. There was heavy fog at the airport, and while the US Airways flight was waiting for takeoff clearance a United aircraft ended up on a runway instead of a taxiway. The controller couldn't see the runways because of the fog and she believed the United aircraft was on a taxiway, so she gave the US Airways flight clearance to take off even though the United pilot said several times that they were on a runway.

The US Airways pilot had heard the discussion (actually, almost a debate) between the controller and the United pilot, and he refused the takeoff clearance because he wasn't confident that the situation was under control. It's well worth watching the whole thing, but the key exchange is this:


  ATC: [...] USAir 2998, runway 5R, fly runway heading, cleared for
  takeoff
  
  US2998: Uh, tower, USAir 2998, 'til we figure out what's going on down
  there, we're just gonna stay clear of all runways


This level of awareness is expected of all pilots, including light aircraft pilots at uncontrolled fields. Although in that situation there's the additional complication that there may be aircraft around that don't even have radios!