Consider drafting a quick table or grid when you have a test with multiple factors to consider.  It can be easier to find missing combinations in a table, especially if you don't focus on the specifics too closely as you recheck. Ask the question, "Have I covered everything?"

Sometimes links and buttons don't really register with us when we look at them. When's the last time you clicked a help button? Can't remember? We forget about them sometimes. I recently crashed a portlet by clicking help. It happens. When you see a button, click it.