I don't know what this means.. hope it helps!

111x Usage: self:SetVertexColor(color [, a])
[string "=[C]"]: in function SetVertexColor' [string "@SharedXML/Backdrop.lua"]:430: in function SetBackdropBorderColor'
[string "@Plexus/Indicators/Icon.lua"]:183: in function Reset' [string "@Plexus/Frame.lua"]:101: in function ResetAllIndicators'
[string "@Plexus/Frame.lua"]:1399: in function UpdateIndicators' [string "@Plexus/Frame.lua"]:1356: in function ?'
[string "@BugSack/Libs/CallbackHandler-1.0-7/CallbackHandler-1.0.lua"]:119: in function <...ack/Libs/CallbackHandler-1.0/CallbackHandler-1.0.lua:119>
[string "=[C]"]: ?
[string "@BugSack/Libs/CallbackHandler-1.0-7/CallbackHandler-1.0.lua"]:29: in function <...ack/Libs/CallbackHandler-1.0/CallbackHandler-1.0.lua:25>
[string "@BugSack/Libs/CallbackHandler-1.0-7/CallbackHandler-1.0.lua"]:64: in function SendMessage' [string "@Plexus/Status.lua"]:455: in function Reset'
[string "@Plexus/Core.lua"]:800: in function `ResetModules'

Do you know what was happening at the time the error occurred?

Sorry I don't, it might be from a while ago, I just happened to check bugsack before logging off and saw it. Willl post if it happens again.