Not exactly sure when this gets triggered, but it happens pretty consistently with the latest version:

35x ...PetTracker/libs/Sushi-3.1-5/classes/drops/Dropdown.lua:148: calling 'IsMouseOver' on bad self (Usage: local isMouseOver = self:IsMouseOver([offsetTop, offsetBottom, offsetLeft, offsetRight]))
[string "=[C]"]: in function `IsMouseOver'
[string "@PetTracker/libs/Sushi-3.1-5/classes/drops/Dropdown.lua"]:148: in function <...PetTracker/libs/Sushi-3.1/classes/drops/Dropdown.lua:147>
[string "@PetTracker/libs/Sushi-3.1-5/classes/drops/Dropdown.lua"]:153: in function <...PetTracker/libs/Sushi-3.1/classes/drops/Dropdown.lua:147>
[string "=(tail call)"]: ?
[string "@PetTracker/libs/Sushi-3.1-5/classes/drops/Dropdown.lua"]:93: in function <...PetTracker/libs/Sushi-3.1/classes/drops/Dropdown.lua:92>

Locals:
(*temporary) = SimpleCheckout {
0 =
RightOutside = Line {
}
RightInside = Line {
}
Background = Frame {
}
CalculateDesiredSize = defined @Blizzard_StoreUI/Blizzard_SimpleCheckout.lua:101
OnShow = defined @Blizzard_StoreUI/Blizzard_SimpleCheckout.lua:54
OnLoad = defined @Blizzard_StoreUI/Blizzard_SimpleCheckout.lua:25
OnHide = defined @Blizzard_StoreUI/Blizzard_SimpleCheckout.lua:61
LeftOutside = Line {
}
OnExternalLink = defined @Blizzard_StoreUI/Blizzard_SimpleCheckout.lua:83
TopInside = Line {
}
BottomOutside = Line {
}
CloseButton = Button {
}
TopOutside = Line {
}
BottomInside = Line {
}
OnEvent = defined @Blizzard_StoreUI/Blizzard_SimpleCheckout.lua:29
LeftInside = Line {
}
RecalculateSize = defined @Blizzard_StoreUI/Blizzard_SimpleCheckout.lua:139
OnRequestNewSize = defined @Blizzard_StoreUI/Blizzard_SimpleCheckout.lua:75
}

It's happening to me when I'm in the new vault area and the Recipe Rat vanishes from my bags. Might trigger when an object leaves your bags through non-standard methods.

I had this pop up today while flying in Icecrown, but I haven't been able to repro it.