Luckily I have never fallen all the way through before. Come close, one particularly hairy moment was on the top floor of a mill a couple of years ago, the majority of the floors were absolutely sound but in one area water damage and general decay had taken hold, and as I walked across the 'solid' floor (covered in old grain debris and very dark) I heard a sharp 'crack' sound and the floor sink beneath me, needless to say I used ninja skills I didn't know I had and jumped back quicker than anything I've done before and all was well...close one though!

I have fallen through the floor in an 100 year old abandoned theatre before. It was about a 12 foot fall but thankfully the lobby under the floor was covered in a pile of mushy, wet plaster and drywall which broke my fall and prevented me from being injured beyond bruises and cuts. I had that terrifying "near death" moment though since I wasn't sure what I was going to fall onto.

The floor didn't give any signs that it could collapse. No creaking or movement what so ever. Sometimes you just don't know.

This floor looked pretty bad.
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A small group of friends and I were climbing a very tall spiral staircase in an old grain silo once when I suddenly fell halfway into the stairs. It turns out that the stairs were a metal frame with some sort of material placed inside. One of the steps was missing that material, and my foot went right through. Thankfully it was only one foot and I came out unscathed, but it was quite a scare for everyone. Be safe out there! Don't forget to look down.