If you try to add a cell into an existing group via Ctrl-C Ctrl-V, the original group breaks up in a non-reparable way (in the Manual grouping regime). The cells are neither grouped anymore nor ungrouped. An attempt to just group everything together produces a beep. Only deleting inserted cell helps.
Version 12.0.0.0 of Mathematica, but was present in previous versions as far as I remember.
See the picture before and after Ctrl-V 
