Subsequent clearances
Let me add me tuppence worth here.
EXAMPLE:
"Descend FLAAA" level By ABC then later....
"Descend FLBBB"...
Second clearance technically does cancel the first one - however, in my opinion you'd be a bit of a dick to ignore it. The original restriction was in there for a reason, not for the entertainment of the controller.
Yes, it might be an oversight by the controller who should really have said;
"descend FLBBB, cross ABC FLAAA or below"
But would you deliberately ignore it and possibly put yourself in a dangerous position, just to prove a point?.................