Vael
Legend
One time a player was attacking a T-Rex by throwing shurikens at it. Whenever that player would miss I would say, "you hit it, and it doesn't care!" because I imagine a decently leveled character could hit the board side of a barn (which is sized similar to a T-Rex). However, making the shuriken hit a point that would hurt the T-Rex is a bit harder...
Same thing happened to me in the last Pathfinder game. We were fighting (actually, trying for a few rounds and then running away, the AC and DR were too high for us) a Stone Golem, and while I wasn't rolling badly, I couldn't actually hit the Golem's AC. The GM narrated it as my arrows pinging off the Golem's body. Meanwhile the Druid's animal companion was "hitting", as in rolling high enough, but not penetrating DR.