(tested with no other addons enabled)
Simply typing /ipopbar in chatwindow (to show default help) generates this error:

Message: Non-critical error: 'Change the time it takes before the popbar rows disappear. Only affects combat. X can be between 0 and 5 (0.2 default).' is not defined in the IPopBar localization tables.
Time: 11/09/09 02:40:06
Count: 1
Stack: [string "Interface\FrameXML\BasicControls.xml:<Scrip..."]:18: in function <[string "Interface\FrameXML\BasicControls.xml:<Scrip..."]:4>
Interface\AddOns\IPopBar\Locales\Locale-enUS.lua:105 in function <Interface\AddOns\IPopBar\Locales\Locale-enUS.lua:104
Interface\AddOns\IPopBar\IPopBar.lua:632 in function <Interface\AddOns\IPopBar\IPopBar.lua:504
Interface\AddOns\IPopBar\IPopBar.lua:887 in function `value'
Interface\FrameXML\ChatFrame.lua:3529 in function `ChatEdit_ParseText'
Interface\FrameXML\ChatFrame.lua:3177 in function `ChatEdit_SendText'
Interface\FrameXML\ChatFrame.lua:3201 in function `ChatEdit_OnEnterPressed'
[string "*:OnEnterPressed"]:1: in function <[string "*:OnEnterPressed"]:1>

Locals: msg = ""
quietmode = nil
(*temporary) = <function> defined =[C]:-1
(*temporary) = ChatFrame1 {
 0 = <userdata>
 flashTimer = 0
 isInitialized = 1
 tellTimer = 50777.987
 oldAlpha = 0
 channelList = <table> {
 }
 init = 2
 isDocked = 1
 editBox = ChatFrameEditBox {
 }
 isLocked = 1
 name = "General"
 hoverTime = 0
 buttonSide = "left"
 zoneChannelList = <table> {
 }
 defaultLanguage = "Common"
 messageTypeList = <table> {
 }
}
(*temporary) = ""
(*temporary) = "/ipopbar"
(*temporary) = " disappear X - "
(*temporary) = <table> {
 Click to toggle IPopBar. = "Click to toggle IPopBar."
 Show/hide the action page number on the latency meter = "Show/hide the action page number on the latency meter"
 The amount of time before the popbar rows disappear. Only affects combat. = "The amount of time before the popbar rows disappear. Only affects combat."
 Scale the main menu bar. X can be between 0.5 and 2.0. = "Scale the main menu bar. X can be between 0.5 and 2.0."
 Reset to defaults = "Reset to defaults"
 Invalid ID specified. It must be between 1 and 110. = "Invalid ID specified. It must be between 1 and 110."
 Toggle Menu Bar/IPopBar = "Toggle Menu Bar/IPopBar"
 Invalid time specified. It must be between 0 and 5.0. = "Invalid time specified. It must be between 0 and 5.0."
 IPopBar cannot change this setting while in combat. = "IPopBar cannot change this setting while in combat."
 Change the time it takes before the popbar rows appear. Only affects combat. X can be between 0 and 5. (0.2 default) = "Change the time it takes before the popbar rows appear. Only affects combat. X can be between 0 and 5. (0.2 default)"
 The amount of time before the popbar rows appear. Only affects combat. = "The amount of time before the popbar rows appear. Only affects combat."
 Change the time it takes before the popbar rows disappear. Only affects combat. X can be between 0 and 5. (0.2 default) = "Change the time it takes before the popbar rows disappear. Only affects combat. X can be between 0 and 5. (0.2 default)"
 Main bar scaling = "Main bar scaling"
 IPopBar is now set to only use %d row(s) of buttons. = "IPopBar is now set to only use %d row(s) of buttons."
 Auto-switch to bar mode on combat = "Auto-switch to bar mode on combat"
 Resets the starting action IDs of all the rows to the defaults. = "Resets the starting action IDs of all the rows to the defaults."
 Hover out time = "Hover out time"
 Automatically switch IPopBar to bar mode on entering combat = "Automatically switch IPopBar to bar mode on entering combat"
 Row %d = "Row %d"
 Show dragon end caps = "Show dragon end caps"
 Technical Information = "Technical Information"
 IPopBar Button %d/%d = "IPopBar Button %d/%d"
 ADVANCED_DESC = "This section is for advanced users to setup IPopBar buttons to use a specific range of action IDs on each bar.

The WoW servers store 120 action IDs corresponding to your spells and items on your action bars, the default UI bars uses action IDs 1-72, with some extra action IDs (typically 73-84) for the various forms