Wow, I just got a similar error while testing out the breakpoint/debug function on a script of mine. I right clicked on the red circle on the side, and then accidentally left clicked on the same red circle and this error came up. Easily reproducible, but silly to do.
************** Exception Text **************
System.Collections.Generic.KeyNotFoundException: The given key was not present in the dictionary.
at System.ThrowHelper.ThrowKeyNotFoundException()
at System.Collections.Generic.Dictionary`2.get_Item(TKey key)
at ScriptUO.BreakpointBookmark.Click(Control parent, MouseEventArgs e)
at ICSharpCode.TextEditor.IconBarMargin.HandleMouseDown(Point mousePos, MouseButtons mouseButtons)
at ICSharpCode.TextEditor.TextArea.OnMouseDown(MouseEventArgs e)
at System.Windows.Forms.Control.WmMouseDown(Message& m, MouseButtons button, Int32 clicks)
at System.Windows.Forms.Control.WndProc(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
************** Loaded Assemblies **************
mscorlib
Assembly Version: 2.0.0.0
Win32 Version: 2.0.50727.1433 (REDBITS.050727-1400)
CodeBase: file:///c:/WINDOWS/Microsoft.NET/Framework/v2.0.50727/mscorlib.dll
----------------------------------------
ScriptUO
Assembly Version: 0.0.0.0
Win32 Version: 0.0.0.0
CodeBase: file:///C:/My%20Docs/UO%20stuff/ScriptUO_v195.exe
----------------------------------------
System
Assembly Version: 2.0.0.0
Win32 Version: 2.0.50727.1433 (REDBITS.050727-1400)
CodeBase: file:///C:/WINDOWS/assembly/GAC_MSIL/System/2.0.0.0__b77a5c561934e089/System.dll
----------------------------------------
zip
Assembly Version: 0.0.0.0
Win32 Version: 0.0.0.0
CodeBase: file:///C:/My%20Docs/UO%20stuff/ScriptUO_v195.exe
----------------------------------------
ScriptUO
Assembly Version: 1.0.3172.20198
Win32 Version: 0.0.0.0
CodeBase: file:///C:/My%20Docs/UO%20stuff/ScriptUO_v195.exe
----------------------------------------
System.Windows.Forms
Assembly Version: 2.0.0.0
Win32 Version: 2.0.50727.1433 (REDBITS.050727-1400)
CodeBase: file:///C:/WINDOWS/assembly/GAC_MSIL/System.Windows.Forms/2.0.0.0__b77a5c561934e089/System.Windows.Forms.dll
----------------------------------------
System.Drawing
Assembly Version: 2.0.0.0
Win32 Version: 2.0.50727.1433 (REDBITS.050727-1400)
CodeBase: file:///C:/WINDOWS/assembly/GAC_MSIL/System.Drawing/2.0.0.0__b03f5f7f11d50a3a/System.Drawing.dll
----------------------------------------
Microsoft.VisualBasic
Assembly Version: 8.0.0.0
Win32 Version: 8.0.50727.1433 (REDBITS.050727-1400)
CodeBase: file:///C:/WINDOWS/assembly/GAC_MSIL/Microsoft.VisualBasic/8.0.0.0__b03f5f7f11d50a3a/Microsoft.VisualBasic.dll
----------------------------------------
ICSharpCode.TextEditor
Assembly Version: 0.0.0.0
Win32 Version: 0.0.0.0
CodeBase: file:///C:/My%20Docs/UO%20stuff/ScriptUO_v195.exe
----------------------------------------
System.Xml
Assembly Version: 2.0.0.0
Win32 Version: 2.0.50727.1433 (REDBITS.050727-1400)
CodeBase: file:///C:/WINDOWS/assembly/GAC_MSIL/System.Xml/2.0.0.0__b77a5c561934e089/System.Xml.dll