Bad Day, or Just Bad Math?

I honestly do not know what happened today. I held steady on both leg exercises, and picked up a rep on Bicep Curls. (I now do those immediately after Squats, because the weight is close to the same -- I just remove 20 pounds, then take the weight completely off after Curls, so as not to dimple the wheels.) Then I tried Shoulder Press, with the same (or so I thought) 46 pounds I'd had on it last time.

I could not get the platform to move. At all! I mean, I was actually sliding on the vinyl surface of the platform. That didn't happen last time, and I was wearing the same kind of clothing, sweat pants and a t-shirt. I pulled off a plate on either side, reducing it to 29 pounds, and managed six reps.

Then, I added those two plates back, plus another 15 pounds per side, to make the same 76 pounds I used last time for Pullups. And I could barely manage a single rep! I took off two five-pound plates, but still only got three reps done. I started thinking that, damn, I'm having a really bad upper body day. Except I was fine with Bicep Curls, and although I dropped a rep on Flies, I held steady with eight Dips. Two down on Pullovers, though, and so I am left wondering just what happened.

The weirdest was the Shoulder Press, though. I did five of them last time, and could not move the thing at all this time. Not even a partial, abortive rep -- nothing. It will be interesting to see how I do next time. Will I get it back? Or did I never actually have those 46 pounds on there to begin with? "Bad at math" has never been one of my distinguishing characteristics!