Numpad4 takes screen shots through steam when not assigned to anything.

[GEN.Fire Bird]

/title.
Halp.

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UPDATE 2013/12/17:

Known issues related to Steam:

• ??? Users report that random keys (z, numpad enter, escape) will get stuck as the steam screenshot key.  I haven't been able to duplicate this but perhaps it is related to the above keyboard error?  Is this a bug with steam?  More info needed. 
  

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[GEN.Fire Bird]

Ah. Didn't notice that.
Well, looks like I'm stuck with the arrow keys for ghost block movement.

[TheTurtorn]

Try setting the screenshot key to something you will never use. iirc, random keys are making screenshots if there is none specified in steam.