I recently started to experience strange behavior in Calc. When I copied cells and pasted them, it sometimes did not work correctly. Worse, when I tried to do a second paste, formulas consistently got lost and I got only the cell values.
It turns out xfce4-clipman is the culprit. This is a plugin that keeps a history of your selections. According to the manual, it is a good idea to check the option Sync selections. Turning it off, though, seems to solve this problem.