I haven't seen it happen for "space" reasons (although I have seen it happen for "size" reasons) but how often (if at all) and to what extent (e.g. 1 bag on a flight? 10 bags on a flight?) do you more frequent travellers see passenger carry-on baggage being checked due to space concerns (i.e. bins too full)? It would seem to me that the "space shortage" would, on any given flight, not be so large that it couldn't be sorted out through some reshuffling (e.g. get some passengers to put their laptop bags, etc. under the seat in front of them).