The Canadian cup is 227mL, while the US cup is 236mL, and the metric cup is 250mL. So good fucking luck figuring out which one your recipe uses. Luckily, for most cooking it doesn't really matter to be off by 9-23mL.

If I'm scaling a recipe up or down, I have to convert cups/spoons to metric, do the math, and then convert back. I can't remember how many hogsheads to a dram.