An interesting question was asked about clearing the contents of a custom lookup field when URL hacking the clone button on an opportunity. The question was deleted by the asker, but the behavior is a little odd, so I thought it was worth reasking/answering.
Attempt: Use the Custom Field Id in the URL and set it equal to nothing.
Result: When creating the record the field is blank if the user populates the field all is well in the world. If the user leaves the field blank the previous value reappears.