At midnight on the night before I was to give a talk at a conference a couple of years ago, I was going through my transparencies and practicing the talk one more time when I realized that the graph was mislabeled. (If I remember correctly, the color code for different curves on this particular graph was reversed from the color-to-curve schema in the other graphs, and the legend had labeled the curves according to the other graphs' color assignments.) Since I was using transparencies specifically so I wouldn't have to bring a laptop, I couldn't change the legend in a file on said laptop.
Oh noes!

Not to worry, however--I casually held my hand over the graph legend when I put the transparency on the overhead projector, and I said the proper labeling out loud when pointing to the plot.
Nobody said a thing.