I've been arachnophobic since I was about 7, and a huge one dropped out of a light onto the top of my head. My sis took one look at it and ran screaming, and I didn't know what it was, but could feel it starting to crawl down onto my face. Thankfully my bro flicked it off my head, and when I saw it on the floor, it was HUGE and hey presto, welcome to arachnophobia! The worst thing was no-one else in the house was bothered enough to get rid of it, so I kept coming across the darn thing! Awful now I think back, what were my parents thinking!!??
It was quite harrowing for me, as we lived in the country, spiders were always around. Then I moved to live in Brisbane, Aust for 10 years, and to begin with my phobia was really exacerbated huge huntsmans all over the place, but then when I had kids, something quite remarkable happened one night. I had two children asleep in the room, and 2 large huntsman spiders sitting on the wall. I was quite sleep deprived, and my options were to chase the spiders out, and risk waking the kids, or just forget about them. Amazingly enough, I managed to get to sleep, by morning the spiders had gone elsewhere, and I am SO much better with them now.
De-sensitisation really does work, it's not a situation I would ever have put myself in willingly, but I am glad it has helped my phobia to ease so much.