I dunno.....how many times you opened a can of PVC glue to do a few fittings, then come back to that can in a few months to find it set up (or dang near it), even though the lid was tight. I try to buy it in the smallest cans I can find unless I know I'm doing a pretty big project, and I've still wasted a lot of it over the years.
Of course, the flip side of that is "how many cans of spray paint have you thrown away because the push tip clogs from the previous use ?" and my answer would be "yeah, a lot of them"
I do like the anti-Murphy's Law of Statistics feature built into spray cans versus open cans....that law being "Any open can of glue has a 90% chance of going from vertical to horizontal the minute you set it down"