Definitely NOT the only one who has had this happen. I have replied to many similar threads before.
DS was 3 months old when he was sitting on the couch in his "horse shoe" pillow. (It helped him with sitting up).
I was quickly going to the kitchen to grab a towel to wipe his spew off his shirt (took me all but 10 seconds) and before I knew it, I heard a "THUMP" followed by an earpiercing an heart breaking "WAAAAAHAHAHHAAAHHAHAAAAAA!!!"
I have never ran that fast in my entire life
I was glad I choose to have carpet laid over the tiles when I bought this house. Im sure that had he fallen on tiles, something bad would have been the result.
He was ok though, but I was kicking myself for weeks after.
So no, not the only bad mum. We're only human and yes "cliche" accidents happen