When double clicking a column separator to resize the column to fit the contents, sometimes it takes multiple double clicks for the resize to actually happen.  After the column finally resizes, sometimes the keyboard input no longer works to change the contents of a cell (although you can still click in the formula bar and type in there), and the arrow keys also no longer work to navigate through the spreadsheet (although you can still change the active cell by clicking).  The only way I've found to recover the use of the keyboard is to save/exit and restart gnumeric.

This is a longstanding issue, and it is easily reproducible on Fedora's gnumeric build at least.