Will have been braun or luer-lock or something mate. Not cheapskate ones. Only ever happened once in the 17yrs ive been using and it was about 2 yrs ago with some tren i think? After it happened i found that others had had the same thing happen. I think the difference between the stopped in a vial and that in a syringe is the vial is usually silicone which is less reactive than ordinary rubber.