What steps will reproduce the problem?
        local row = tooltip:AddLine()
        tooltip:SetCell(row, 1, label, nil, nil, 3)
        tooltip:SetCell(row, 4, hint)
        tooltip:SetLineTextColor(
            row, NORMAL_FONT_COLOR.r, NORMAL_FONT_COLOR.g, NORMAL_FONT_COLOR.b)

What is the expected output? What do you see instead?
Message: Interface\AddOns\LibQTip-1.0\LibQTip-1.0.lua:1097 attempt to index field '?' (a boolean value)
Time: 07/27/13 09:54:13
Count: 1
Stack: [C]: ?
Interface\AddOns\LibQTip-1.0\LibQTip-1.0.lua:1097 in function `SetLineTextColor'
Interface\AddOns\KnowYourLootSpec\src\Tooltip.lua:38 in function <Interface\AddOns\KnowYourLootSpec\src\Tooltip.lua:33
Interface\AddOns\KnowYourLootSpec\src\Tooltip.lua:62 in function `RefreshTooltip'
Interface\AddOns\KnowYourLootSpec\src\Tooltip.lua:28 in function `ShowTooltip'
Interface\AddOns\KnowYourLootSpec\src\DataBroker.lua:13 in function `OnEnter'
Interface\AddOns\Bazooka\Bazooka.lua:1431 in function `showTip'
Interface\AddOns\Bazooka\Bazooka.lua:1246 in function <Interface\AddOns\Bazooka\Bazooka.lua:1237

What version of the product are you using?
1.0, minor 42

Do you have an error log of what happened?
see above

Please provide any additional information below.
line.cells[cell_index] contains the boolean "false" for cells that are skipped because a previous cell spans over it, so the line in this loop body throws the exception:
    for cell_index = 1, #line.cells do
        self:SetCellTextColor(lineNum, line.cells[cell_index]._column, r, g, b, a)
    end