I photographed a car rally yesterday. I'm happy with the results, but because I was constrained by time and buggered knees, I had to make the most of a single location, not far from my vehicle. Consequently, I geared up to produce as many different images as i could from that one spot. I had my 100-400 bolted to the 7D2 and my 24-70 on the 5D3. I then walked a little way downstream of a 90 degree corner, on a straight. I used the 100-400 to shoot cars exiting the corner, then quickly swapped to the 24-70 to pan the cars as they passed by.
That was the plan.
However, being in a forest, the trees came right up to the track, which meant I was quite close. "That's OK" I thought, "The 24-70 is wide enough and I'm a wiz at panning."
For those who haven't tried panning fast-moving objects, being close to very fast-moving subjects is tricky because you're going to have to swing the camera pretty quickly. This means there's lots of scope for vertical errors and poor subject tracking. I had done this plenty of times before though, so I was confident I would get some good pics.
But brain fade set in. I selected a slow shutter for good blur - 1/50sec, which is what I would normally use. When operating so close to 100mph cars however, you don't actually need to go that slow because you're swinging the camera really quickly and that will give decent blur even with 125/sec or thereabouts, while reducing the impact of errors forementioned.
So I stuffed up. It wasn't till I'd shot every car on its first pass that I saw I wasn't getting many keepers, and realised my error. I dialled in 1/125sec and Bingo!
Note to self: you know what to do; you just have to remember to do it, ya berk. 
5D3, 7D2, EF 16-35 f/2.8L, EF 24-70 f/2.8L II, EF 24-105 f/4L, EF 70-200 f/2.8L II, EF 100-400 f/4.5-5.6L II, EF 1.4x III, Sigma 150mm macro, Lumix LX100 plus a cupboard full of bags, tripods, flashes & stuff.
