In high school I once got a reading of 0.6 mmol/L, which is 11 mg/dl. I checked the meter manual (many years later, it was a Dex at the time) and found that it actually does read that low. At the time I definitely felt low but I wasn’t having a seizure or anything! I think the meter may have been off, though, because I find it hard to believe I could be that low and still walking and talking and conscious. As a kid and into adolescence I had hypoglycemia unawareness, however, and could have very low readings without feeling off at all. Thankfully, that’s gone away with newer insulins and the switch to better regimens as I got older.
Other than that the lowest I’ve had is 1.2 mmol/L, which is about 21 mg/dl. For this one I felt perfectly fine, though my mom freaked out. I’m also pretty sure I’ve gotten a couple of “LO” readings, or too low for the meter to read (usually below 1.0 mmol/L, 20 mg/dl). I also had several severe lows as a kid where I was semiconscious and people did not bother checking my blood sugar before giving me something, so who knows how low I was those times.