What is the problem? Is there any Lua errors?
Lua error when enabling/disabling Castbar Nameplate

1x ...\ClassicCastbars_Options\ClassicCastbars_Options-v1.4.0.lua:90: attempt to call method 'DisableBlizzardCastbar' (a nil value)
...\ClassicCastbars_Options\ClassicCastbars_Options-v1.4.0.lua:90: in function <...\ClassicCastbars_Options\ClassicCastbars_Options.lua:87>
[C]: ?
...nfig-3.0\AceConfigDialog-3.0\AceConfigDialog-3.0-81.lua:51: in function <...nfig-3.0\AceConfigDialog-3.0\AceConfigDialog-3.0.lua:49>
...nfig-3.0\AceConfigDialog-3.0\AceConfigDialog-3.0-81.lua:847: in function <...nfig-3.0\AceConfigDialog-3.0\AceConfigDialog-3.0.lua:668>
[C]: ?
BagSync\libs\AceGUI-3.0\AceGUI-3.0-41.lua:72: in function <BagSync\libs\AceGUI-3.0\AceGUI-3.0.lua:70>
BagSync\libs\AceGUI-3.0\AceGUI-3.0-41.lua:306: in function `Fire'
...nc\libs\AceGUI-3.0-41\widgets\AceGUIWidget-CheckBox.lua:68: in function <...nc\libs\AceGUI-3.0\widgets\AceGUIWidget-CheckBox.lua:57>

Is this for Classic or TBC client?
Classic ERA

It's happening while deactivating the castbar nameplate.
so, it's a very minor annoyance.