From these pictures it's difficult to tell whether or not the dent is accidental or intentional.

A couple of points:

- If the dent is very symmetrical (if it looks like it was deliberately made) then you're probably fine.

- If the dent is actually behind the crankset, how could it have been accidentally created? Unless you, or someone else, stripped the frame completely before a dent was accidentally created, I don't see how it's possible for accidental damage to have occurred to this location on the frame

- It's a steel frame, and as noted before, steel is forgiving of minor damage. The dent appears to be on a thick area of the chainstay reinforced by the chainstay bridge. I doubt you have anything at all to worry about.

Ride on, I say!