When you're using Firefox on Ion, it doesn't notice when you switch windows so your typing all goes to the original window. After two years of pestering, it was tracked down and fixed in GTK 2.6.8, released last month: "Fix a bug in focus tracking that lead to problems between some window managers and GTK+ applications." (X geeks: The bug report contains a detailed description of the situation.)
Ahh, sweet relief. I was so annoyed by this that I had been considering giving up Ion.