so i found a shot i must of stashed away a few days ago.. and it looked like part of it solidified.. but when i shot it out into a spoon it was a clear liquid.. so i refiltered and shot it.
If you heat your shot, cut will break down when heated, and re-solidify after sitting at room temp. That's why I do mine cold. I've had this happen to me before.
Bacterial growth seems to be the main concern. You've had it exposed to contamination on the spoon and then sucked it up into a syringe and left it sitting around presumably at room temperature for a few days. I think a quick boil back on the spoon then micron filtering would be the way forward if you must use an old shot. In an ideal world you'd bin it but we know that ain't likely to happen, is it?
Leaving Heroin in solution will degrade it. Heroin will break down by hydrolysis to first 6-MAM and then to Morphine in water. I'm not sure that's a problem if you catch it at the 6-MAM stage because that's exactly what happens in the body once you've injected, Heroin being a prodrug, and it's the 6-MAM and Morphine that are responsible for the effects, 6-MAM being the stronger.
As an aside, in the UK it's quite common to see a shot solidify as it cools because we have to use citric acid to dissolve the H. I've watched citric recrystallise out in front of my very eyes. Many a time had to warm the syringe with a lighter to redissolve the crystals so I could get it back out onto a spoon and try again with a bit more water, sometimes needing to cut the end off and push out something that looked a lot like a Heroin Slush Puppy!