The last little bit on tin can ring pull lid which never comes off and when pulled send splatters of the contents all over the kitchen and you. Especially pleasant when it's tuna or dog food
Absolutely Lardman, especially tins of sardines in sunflower oil which double the jeopardy as it's difficult to fully open the tin without the outside getting very slippery!
(Hard to compute the overall health benefits of eating an omega-3 rich food with the possibility of needing a trip to A&E with a severed finger!)
'Resealable' bags which aren't are another minor aggravation (hard to open the bag without causing a far bigger split than can be covered with the 'handy resealing strip', which usually isn't sticky enough to do the job even once anyway!