Toolbar's URL and Google search text fields disappeared

Today the two text fields in the toolbar for the URL and Google search went away. There was a space there, but no place to enter a URL or type in a search term. It was just the background gray. Selecting "Hide Toolbar" and then "Show Toolbar" did not help. I shutdown the machine and restarted it, but it still did not help.

Finally I selected "Customize Toolbar..." and added the "Add bookmark, Address, and Search" item (the stuff that disappeared) to the toolbar. This solved the problem.

Anyone know what I did to cause these fields to disappear? Was a file corrupted? How to I prevent this from happening in the future?