An irritating feature in LO Calc — it’s been there as long as I remember, still there as of version 5.0.1.2 (in English, on OS X) — is that after copying a cell or a range of cells to the clipboard, the selection becomes… “highlighted” for the lack of a better term (distinct from being selected; visually identified by an animated dashed border). After this, as long as no cells’ contents have been edited, hitting Enter will paste the clipboard contents into the currently selected cell(s). This overrides both any other actions assigned for the Enter key, and the usual warning about pasting data into cells that already have content.
I have no use for this feature, and I would like to disable it, but I’ve so far not found anything in the documentation that as much as acknowledges that the feature even exists.