Uh, excuse me - what? If the overhead compartments for carry-on baggage is full - you got plenty of space under the seat in front of you.
The only exception is really if you're by the emergency exits - then you're not allowed to have anything that could obstruct or confuse people when they make their way to the emergency exit.
Well, that came out silly in hindsight - of course there are more options than the overhead compartment and under the chair in front of you.
What I was trying to say is that I've found it unusual that I've not been able to put my carry-on baggage under the chair in front of me and I have not noticed the flight crew having to handle any other options than the two above.
The only exception is really if you're by the emergency exits - then you're not allowed to have anything that could obstruct or confuse people when they make their way to the emergency exit.