Philip K Dick's "Adjustment Team" aka "The Adjustment Bureau" (novelette, free): A man accidentally watches God in action
One of these adjustments went a little wrong, in that it didn't change all that needed changing. One "element" remained unchanged! Element that's a man named Ed Fletcher.
This is the story of Fletcher's nightmare that results.
Fact sheet.First published: Orbit, #4 (September-October 1954).
Download full text from Southern Cross Review or scans from Wikimedia Commons.
Credits: I looked up this story after seeing its mention in David Langford's Ansible, November 2011. Link to scans at Wikimedia is via Best Science Fiction Stories (I personally haven't read these scans).
Related: Stories of Philip K Dick.