Lucky there wasent a flame and the flour / air mixture wasent at critical mass, mills / custard factories and what not have been known to blow up.
As mythbusters so fine illustrated with ther creamer cannon, a lot of things can go wrong with powder in the air and a ignition source.
I have even heard it in small scale where i was working with powdercoating, on the automated line 8 guns was active in 1 booth, and then a spark jumped due to bad grounding of the things hanging on the conveyor above.
The painter in the booth was blown out and down the 1 M or so stair up to the booth, and that in spite of both ends of the paint booth was wide open.