Last night I attempted to make a reduced sauce out of equal parts soy sauce, mirin, sake, and sugar. I let it simmer in a saucepan over medium-low heat until it got to a mildly viscous consistency where dragging my spatula briefly left a trail in the pan, then put it in a jar and put it in the fridge. The next day it was practically solid - I heated it up for 30 seconds in the microwave and it had the consistency of molasses. This was despite it being still pretty runny when I took it off the heat.
I've noticed this has happened a couple of times, and I want to know if there's something I'm doing wrong. Perhaps I should be taking it off the heat much earlier (though at that point there would be no trail left in the pan by my spatula, contradicting what most people suggest).
I also noticed at one point the mixture was bubbling quite a bit - I think I let it get too hot, which promoted crystallization of the sugar. Is it possible overheating the sugar results in too thick a reduction, and one should vigilantly watch their saucepan to prevent it from overheating?