I'm not sure I can narrow it down to just one. I have done them all. Here are my top five:

1. Constantly changing rivers to follow the hot fish reports.

2. Using too heavy line and too large baits and lures. (Many beginners think the larger the lure the better chance a fish will see and eat it.)

3. Not knowing how to read water.

4. Not knowing how to detect a bite.

5. As fun5 said losing concentration. You need to expect a fish with every cast. That can be hard to maintain on cast number 2,368.