For some reason I have always felt compelled to thoroughly wash all my fruits and veggies, including those pre-packaged salads, spinach etc. that claim to be pre-washed. Call me obsessive, call me paranoid, but I just have to do it. Perhaps it's because I have had so many gross experiences in my life with food. I've bitten into a plum and had a worm pop out the other end, right in front of my eyes. The same thing happened with a candy bar, when I still ate chocolate. This big, fat green worm popped up as I bit into a Mr. Goodbar! On another occasion, after having eaten half a box of raisins, I discovered a dead moth and realized (to my horror) why the raisins had been so granular. I've found worms in frozen and canned vegetables, flies in my falafel and hairs in my spaghetti, so you can understand why I am so neurotic when it comes to food inspection and cleanliness both at home and in restaurants.
The following photo confirmed why it is still imperative to wash all those pre-washed salad fixings:
If you look closely below the "Fresco, Lavado" you will see the bulging eyes of a frog. Yummy. The email accompanying the photo assumes that the water the spinach was washed in contained polliwogs, but whether the photo is real or a hoax is undetermined, according to Snopes.com. But there have been other similar cases. Either way, it's probably best to wash everything, unless you want extra protein in your salad!