My truck started to slip today from 0 speed around 40 km.....then it would work find from that point on. When ever I stop and go again it would slip.

I decieded to take the oil pan off and inspect the bottom. When I took it off the filter was laying in the bottom of it.

Is this normal? I only did one other transmission and the filter stayed on the tranny when I removed the pan.

I found grey sludge around the magnet.....looks almost like never seeze. I was wondering if that is normal too. the tranny is a 4R75E from a 2005 F150 5.4 4x4. I thought the oil smelt ok. didn't look burnt and I didn't see a bunch of filings. Might have found one it was hard to tell if it was a filing or dirt from removing the pan.

Has any body herd of the filter coming off and causing slipping at low speeds?

 

The filter was very likely still attached prior to you dropping the pan. The pan actually helps keep the filter "propped" up, basically pinning it in.. When the filter media is saturated with fluid it gets very heavy. Upon removing the pan the filter just dangles from the neck of the filter tube. Without the pan under the filter to support it, it can just slip right out during pan removal, due to the weight of the saturated filter material.... When the pan is bolted up, the filter has no way to "fall down" from the transmission. It's a very tight fit. The filters pick up tube actually rests on the bottom of the pan, at an angle..

Your slip could have been low fluid, or sticking solenoids..
If fresh fluid and a filter does not fix your issue, then testing and possibly replacing the TPS sensor up on the throttle body might be your next possible solution..