In the planning board if the project has a long enough list of Components/Versions, dragging a card to the lower right to expose a hidden Component/Version for dropping the card onto does not activate the vertical scroll. Thus you can't drag and drop to the desired Component/Issue.
Tested in IE 7 and Firefox 2.0.0.7 on Windows Vista 32 bit and Jira 3.9.3 with GreenHopper 1.2.1
This is why in these situations we have added the select box for version/component directly into the card, so that you can change your version/component without having to drag and drop the card.