As most of you have noticed, my work is tiny. I don't go through supplies very quickly; a half-hide of petite tooling calf has been known to last me 3-4 years (granted, during the dry hobby years of college). I still have some of the sample bottles from my very first dye order. Here's the problem: supplies have a shelf life.
This is especially obvious with dyes, though sealants, glues, and apoxy can have similar problems. The older Eco-Flo dyes that I use are easily six years old or older, and as they age they tend to take on a strange, iridescent green-yellow tinge. Some of it can be helped by vigorously shaking the dye before use and even more can be buffed out after the dye has dried, but it's better to just pony up and buy a new bottle when you get to that point.
My primary example for this post, though, is Satin Sheen (or Sheene, if you're Tandy). Here's a photo:
From left to right on the sample strip: a single coat of new satin sheen, old satin sheen, and super sheen (glossy). As you can see, the old bottle of satin sheen (which is probably 6-7 years old) started taking on a gloss finish. GLOSS. More than just the shine of supple, conditioned leather - this is just glossy, and a problem. I let several saddles leave the studio like that and was okay with it, but I picked up a new bottle and it seals MATTE, which makes this girl happy!
Notes on other things that go bad - I don't sculpt often, so my little sample containers of apoxy sculpt went vinegary and eventually didn't cure properly. It's since been replaced, since I do occasionally need to sculpt a one-off tree (and I have plans to sculpt more!). Glues may lose some of their stick-ability over time, though I've had more trouble with infrequently used super glue inevitably gumming up in the nozzle no matter how well it's sealed.
So here's my pro-tip: write a date on your dyes, sealers, and glues. When you see them start to act funny (and you know it's more than occasional user error), put in a new order and jot down the date, so you know what kind of shelf life you can expect.
Personally, I'd date every bottle of dye, since differing colors have differing chemical properties and rates of aging.