Here are a couple that actually happened:

-A woman called us and told us that she became pregnant after using
contraceptive jelly. Turns out she was putting it on toast and eating it...

-A couple called us saying that they became pregnant after using Tri-Cyclin
that our pharmacy dispensed. It turns out that the husband used to take a
pill whenever his wife forgot to take it. He apparently thought that if
either one of them took it, it would be ok.