Did you connect a card (or a database record) to another card using a connection field* and by any reason (you connected it by mistake, you no longer want it to be connected, etc.) want to disconnect it?
Don't worry, deleting a connected card/record is easy. If the connection field is in the phase the card is currently in, simply hover over the connected card/record and click on the "x" in the right side of the card/record.
If the connection field is in a previous phase, click in the pencil icon to edit the field, then in the "x" icon.
*Important: If the card/record was connected using a connection, the only way to unlink/disconnect it is to delete either the parent card or the connected card/record.