After a copy and paste special action, Insert comment, When press [Enter] then paste is performed.
Then undo does not correctly reverse the damage.

To duplicate enter into [Cell]=____
A1= 1
B1= 6
A2= =A1+1
copy A2
highlight A2:B10
paste
highlight A1:B10
copy
move cursor or click to C1
Paste Special, Numbers only
move cursor or click to A4 (Without pressing any other keys)
change background color of cell
right click on cell, click on Insert Comment
type some text, press [Enter] to either add line or accept entry (that's confusing, but a separate concern)
The [Enter] activated the paste, overwriting many cells.
Click outside the comment to finish inserting comment.
Press [Ctrl]-Z to undo.
See the text was removed from the comment box, but the paste is still there, as is the comment box.
Press [Ctrl]-Z to undo again.
The paste was reversed, but the red square is back on A4 indicating a comment is attached. BUT the comment box was moved to left and above A1, it is not attached to a editable cell now.

Notice the comment box attached to A4 will appear when the mouse hovers over the cell.
Press [Ctrl]-Z again, to undo the background color. The cell range of the paste also becomes non-highlighted.
Left click the unattached comment box, see it is now a drawing object.
Right click to open the object menu.
click elsewhere to remove the floating menu.
Drag the box across the sheet, notice the arrow is now pointing to A4.
Hover the mouse over A4, see another (second) comment box.
Press [Ctrl]-Z and the drag is undone. The orphaned box moved back to the upper left.
Press [Ctrl]-Z again, and LibO crashes and disappears. Seems to be removing a bad pointer, wrong memory location to delete.

Either this last bit is a separate bug, or is a result of the first and primary bug being reported. You decide :)

Therefore pressing Enter during insert comment, may result in document loss. May be difficult to get back to before paste.