Regulus mines, betrayer, maim, wyrmskin handguards took me down to 2.6 movement speed (base of 6.0, unbuffed).  This is in the current beta build (haven't tested non-beta), though this is the first I've encountered such slowness since before the game was released.

Their team also had an Unclean beast, but if he was slowing I didn't see any debuffs from him.

Found the source of this bug:

lua\sim\BuffAffects.lua:345
A call to unit:GetMaxSpeed() is made under the assumption it will return the buffed max speed when in reality it returns the buffed & debuffed max speed.  So if you have 6.0 base, buffed to 6.3 and debuffed to 2.4 unit:GetMaxSpeed() will return 2.4 which is then multiplied by 0.66 which gives you a min speed of 1.584, far below the 4.158 that it should have been.

Unfortunately, unit:GetMaxSpeed() calls into agent:GetSpeedMult() which appears to not be present in the LUA (I'm guessing it's an engine function) so I can't say exactly what the fix is, just that that function needs to return max *buffed* speed, not current buffed/debuffed speed.