Fill Handle suddenly became locked on.
 

I was working on my spreadsheet and the fill handle cursor came on and
wouldn't turn off. I don't think that I clicked anything unusual. I wasn't
trying to use the fill handle feature in this particular spreadsheet either.

Unfortunately I had to close the document which caused me to overwrite half
the document. Thank goodness for backups.

Any idea as to why this happened and how to fix it in the future?

Thank you for your responses.