When pasting externally copied text into a Calc spreadsheet, an "Import Options" dialog appears. Pressing "OK" or "Cancel" in this dialog has the same effect - the text is still pasted. (Possibly not all types of paste trigger the dialog? Formatted text copied from Firefox definitely does, however) Originally raised in bug 46077, but the issue solved under that bug appears to have been separate to this problem so I'm re-raising it as a fresh bug.
Markus Mohrhard committed a patch related to this issue. It has been pushed to "master": http://cgit.freedesktop.org/libreoffice/core/commit/?id=bd8be4eae82525434454cc3c7274f7a7a44158c0 don't paste content if user cancels html import, fdo#47393
Markus Mohrhard committed a patch related to this issue. It has been pushed to "libreoffice-3-5": http://cgit.freedesktop.org/libreoffice/core/commit/?id=76f3a63432e70d017760a2087652a1abc9b126d3&g=libreoffice-3-5 don't paste content if user cancels html import, fdo#47393 It will be available in LibreOffice 3.5.2.