If I've gone through more than say, half a tank of something, and just want to switch it up, I tend to pour the excess into a separate, glass bottle (it's the only glass one I have, making it easy to ID) as opposed to risking contaminating the original bottle with juice that might have possibly burned/darkened/etc. If I only took a puff or two and don't like it, I will indeed dump right back into the bottle, usually using a syringe. The glass bottle ends up filled with all various flavors, sometimes I get something interesting when I sample it at the end of a week/month/whatever, other times I just add a half mil of menthol (covers any other flavors) and use it for emergencies when I'm out of juice.