There's a common misconception that critical pilot errors occur only during flight. A surprising number of accidents result from inadequate preflight, like using checklists and forgetting to fasten the cockpit door (above). They're small mistakes that could escalate into a series of big ones. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.