I typically reuse a lot of bottles and dropper caps, just washing them out in between batches. I made 6 30mL bottles of a strawberry cream, but one of the bottles I used had an especially old dropper, which I think is well over a year old. I've gone through two bottles so far of what I made and everything was fine until I reached bottle number 3.

I filled up my Griffin and after vaping for a while noticed the flavor was way off. I thought the coils were burning, but it had a bad rubbery taste to it. I took apart the tank and the coils were covered in small black rubbery specks. I assumed there was something wrong with the Griffin, so I took it apart and washed it out thoroughly. I checked all the o-rings, assuming something was wrong with them but they were all perfectly fine.

I rewicked, primed it and filled it up. I started vaping again and everything seemed okay, but the rubber taste soon returned. I took the Griffin apart again and again the black specks started to build up on the cotton. I started freaking out at this point since I didn't know what it was. I started to look at the bottle and I couldn't tell if there was anything wrong with the liquid since it was a dark amber bottle. I poured the liquid out, and sure enough there were black specks in the liquid sitting in the bottom. I took the rubber dropper out of the cap and started rubbing it with my thumb and the rubber was shaving off to the touch.

I dumped out the bottle, cleaned my tank and redid everything with bottle number 4 and everything is fine now. Not sure if anyone else has run into this, but these rubber droppers do go bad after a while. I have a giant stock pile of bottles and droppers, some of which are well over a year old that I will have to retire. I'm probably going to switch to solid caps for my bottles and only use dropper bottles when I'm about to drip, which I should have already been doing.

TL;DR : DON'T REUSE RUBBER DROPPERS FOR OVER A YEAR, THEY BREAK DOWN INTO YOUR LIQUID AND YOU'LL HAVE A BAD TIME.