Supertasks

Is it perfectly sensible or fundamentally incoherent to speak of completing a task with infinitely many steps?

Thomson's lamp

You are sitting comfortably in your study reading in the twilight. It is beginning to become dark, and so you turn on the light. But after a little while, you realize the brightness of the lamp has ruined the twilight mood, and so you switch it off. But now it is becoming truly too dark to read, and so you turn it back on again. But of course it is still too bright, so you quickly turn it off, and then on again, and off, ever more rapidly in the vacillating panic of your indecision.