On a recent inner-European flight that was almost or completely booked, many of the passengers brought the typical trolley suitcases that are increasingly used as hand luggage, presumably (based upon visual judgement) within allowable size limits as hand luggage1. The overhead compartments were completely filled, and I witnessed how cabin staff asked various of the passengers who were still looking for a space for their trolleys towards the end of the boarding phase to store their trolleys in front of their seats like this:
No other storage space was available at that point according to staff.
Needless to say, this removed most of the legroom the respective passengers had (depending on shoe size, maybe forcing them to invade their neighbour's legroom), made the part of the legroom below the seat in front of them inaccessible, and (probably most importantly) also made accessing the life vest impossible.
This made me wonder: At this stage of boarding, i.e. all passengers have left the terminal, most are in the cabin, or the doors have even already been closed - is there anything that can be done about hand-luggage for which no space is left?
Please note that I do not consider this a list question. I expect there to be either a very small set of options that would generally apply, or just the statement that it entirely depends on the airline, location, etc. (which would be a valuable answer in itself, too).
As a last detail, the plane was a smaller passenger plane, maybe an Airbus 319 or similar. Larger planes might offer more leeway for stowing away things.
1: Roughly like the ones shown in that other question.
EDIT: Seeing some of the answers/comments so far, please do note that I am not asking about the legality of the luggage storing solution that I observed. I am explicitly asking about realistic short-term alternatives when chances are you end up with your carry-on luggage, but without any cabin space for it.