One time I accidentally got lead-free solder, didn't realize it and kinda freaked out like that
Got through two boards and couldn't figure out why all of my joints looked so damned shitty. Cleaned and changed tips, changes temps, busted out the flux pens- nothin.. Inspected the roll a lil closer... "oh.... oooohhh"
The little carton it came in didn't mention lead free, and it looked like the stuff I normally grabbed at quick glance. Luckily, both circuits worked fine