If you are looking for true rookie mistakes, so here are a few I have seen and may have actually done at one point.
1. Reeling for ever on a fish and then realizing the drag is set way too loose. Then the opposite of setting a drag way to tight and having a fish snap your line.
2. Fighting a snag, line in the water or another fisherman. When the springers were in at the barrier dam it was fairly common to catch a line of someone on the other bank. For a little fun you could slowly strip line out interspering it with a few tugs. The guy across the river swear he had a fish on.
3. Getting excited over a fish only to have it be a carp or dogfish, or even worse a starfish.
4. Forgetting the boat plug.
5. Tossing out of sequence at blue creek and getting everyone pissed off.
6. Thinking 8 pound test is enough to fish for kings in a big river off the bank.
7. Swearing that salmon won't bite in a river.
8. Bragging about how good the fish are bitting when in fact everone of them was lined.
9. Keeping black chum or salmon, and even showing off the pictures.
10. Keeping a really nice down river fish.
11. Bewing too cheap to buy waders.
12. Drift fishing with twice the lead you need and setting the hook on every rock.
13. Taking a freind or wife steelhead fishing and here them complain about the rain or cold or whatever.
14. misidentifying a fish.
15. Being proud of a 12" coho.
16. And the worst.. Telling someone about your "secret spot"