It seems I get this error all the time in Wintergrasp, not sure if it is only that zone but it is most common there.

[00:49:54] Interface\FrameXML\SecureHandlers.lua:511 Cannot use SecureHandlers API during combat
[C]: ?
[C]: in function `error'
Interface\FrameXML\SecureHandlers.lua:511 in function <Interface\FrameXML\SecureHandlers.lua:503
[C]: in function `SetAttribute'
Interface\FrameXML\SecureHandlers.lua:714 in function `SecureHandlerSetFrameRef'
Interface\FrameXML\SecureTemplates.lua:607 in function <Interface\FrameXML\SecureTemplates.lua:559
Interface\FrameXML\SecureTemplates.lua:854 in function `SecureGroupHeader_Update'
Interface\FrameXML\SecureTemplates.lua:443 in function `SecureGroupHeader_OnEvent'
[string "*:OnEvent"]:1: in function <[string "*:OnEvent"]:1>
[C]: ?