This might be too subjective but I'm curious about other peoples' experiences. Mods, please close if this is too far into "not a real question" territory.
I see many questions on this site about "what are good resources?" and "where do I learn?", but what are common mistakes that designers make?
Ie., if you're on the train and you see a poster ad on the wall, how often do you think "wow, I can't believe they did that!"? Are there certain mistakes that are more commonplace than others?