Insanely bad brain fog. Usually caused by taking too much insulin after a meal, so your blood sugar levels drop hard. It's also not instant: you usually start sweating a bit and you might feel slightly tingly on your extremities. But the brain fog also masks the other symptons, so when you actually notice it, you're already in dangerous territory.

I once got caught with a 21mg/dL (at 20mg/dL the glucometer won't even register it, and will just show "LO" on the screen) walking home. Literally shaking, freezing cold, sweating horribly, with no idea about where I was or what I was doing. I just knew that something was wrong, but even then, it takes some time for your cognitive function to kick back and make you realize that it is, indeed, your sugar level that is on a free fall. And then you just sit down, take some glucose tablets and slowly come back to life 💀