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During SpaceX's CRS-1 mission, "engine 1" failed, but the booster still completed its primary mission due to a plentiful thrust and fuel margin.

Clearly, an outboard engine failure is more critical than a center engine failure because it causes asymmetric thrust in addition to reduced thrust, and thus requires gimballing or reduction of the engine opposite it, with further reduced thrust as consequence. CRS-1 used a Falcon 9.0 booster with engines placed in a 3×3 orthogonal grid (rather than today's octaweb) which means that a corner engine failure would cause even more severe asymmetry than a side engine failure.

It is therefore interesting to know the position of the failed engine. Which engine was "engine 1"?

For reference, the orthogonal 3×3 grid of Falcon 9.0 vs. the octaweb of later versions:

3×3 vs octaweb

Adám
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From the Falcon 9 user guide (the 2009 version), it looks like Engine 1 is a corner engine. It's hard to see in the video though.

F9 engine numbering, engine 1 in the bottom right corner

Hobbes
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    Nice. It appears that the corner engines were further from the center than the current outer engines are, which means that Falcon 9's single-engine-out capability was (inadvertently) tested beyond (current) specs. – Adám Feb 22 '18 at 12:50