What accidents have had crew accustomization to warnings among their causes?
A notable accident was the Helios Airways Flight 522 crash, where the pressurization system was turned off and the pilots ignored the warning horn because they thought it was the takeoff config warning. From the Wikipedia page:


  As the aircraft climbed, the pressure inside the cabin gradually decreased. As it passed through an altitude of 12,040 feet (3,670 m), the cabin altitude warning horn sounded. The warning should have prompted the crew to stop climbing, but it was misidentified by the crew as a take-off configuration warning, which signals that the aircraft is not ready for take-off, and can only sound on the ground.


The official accident report explains the source of the confusion in more detail (emphasis mine):


  The Board examined the flight crew’s actions to disengage the autopilot and auto-throttle,
  and to retard the throttles upon onset of the warning horn. Given that the expected
  reaction to a cabin altitude warning horn would have been to stop the climb (there was no
  evidence to this effect), the Board considered such actions to signify that the flight crew
  reacted to the warning horn as if it had been a Takeoff Configuration Warning (the two
  failures use the same warning horn sound). [...]
  
  The warning horn was designed to signal two distinctly different situations. The Board
  considered the role of experience in interpreting and reacting to the warning horn. In the
  course of his career, a pilot is generally likely to only hear the warning horn when it is
  associated with a takeoff and a takeoff configuration problem. [...]
  
  Most pilots are not very likely to experience a cabin pressurization problem and the
  associated warning horn at any time during their line flying.


Conclusion: Never use the same warning for multiple situations, if the pilots will only get used to one of them, otherwiese they will ignore the warning for the other situtations!