  The discovery and study of highly transient sources, especially those which
rise to high brightness and then fade to obscurity, has been a major part of
modern astrophysics. Well known examples include supernovae and novae. A
byproduct of the Deep Lens Survey was the discovery of three transients which
varied on a timescale of less than an hour. All three had faint and red
counterparts, the brightest of which was identified with an M star. However,
the remaining two showed hints of an extragalactic origin, one had a spatially
extended counterpart and the other appeared in projection on the outskirts of a
bright elliptical galaxy. If these two sources were really of an extragalactic
origin then the two events represent a new class of exotic explosive
transients. We undertook spectroscopic observations with the Keck telescope and
find the two counterparts are also late type Galactic dwarfs. Our main
conclusion is that flares from M dwarfs constitute a dense foreground fog and
dominate over any plausible class of extragalactic fast transients by at least
two orders of magnitude. Overcoming this fog will likely require dedicated
surveys with careful optimization of target field location, filter(s) and
cadence, pre-search imaging to filter out late type dwarfs and a well planned
rapid followup plan.
