Definitely happens to me. Unfortunately my worst stupid day was on my first GMAT a few years ago. I am an algebra whiz but got completely stuck on a simple question, which will haunt me forever: "Jose throws a baseball at 90 mph. How many feet per second is that?" I got a dozen different answers, and none were in the ballpark (haha), and before I knew it, I'd killed 15 minutes!! Raced through the rest of my test and bombed.
Of course if you're actually at the test center you can't say, "I'm having a stupid day...can I start again tomorrow?" But if that happens in practice, stop for a while and go do something brainless. If you start to mentally click again, go back to studying.
One thing I will say is that if you avoid too many carbs and eat more protein and veggies and fruit, you will have fewer stupid days. The carbs make me mentally foggy.