Oh.

We had a bit of a problem with O-rings at our plant. The supplier of the filter provided O-rings which were about half a millimetre too small. Which was fine when you're filtering water, but when we tried to prove that the filter was good by putting compressed air in, it failed miserably.

Why are O-rings so called? Isn't the normal shape of a ring an O? Why not call them ring seals or something else? Isn't the term "O-ring" a bit of a tautology?

Never mind me, I'm just a computer systems engineer trying to make sense of the mechanical engineering world.