Go punishes instincts that serve you well elsewhere. If you come from chess, capturing feels like progress. If you come from territory games, enclosing feels safe. Both mislead.
1. Chasing captures
Capturing two stones is worth about four points. Taking a corner is worth ten or more. Beginners spend six moves hunting a small group while the opponent quietly takes the board. Before chasing, ask what the capture is worth compared with the biggest open point.
2. Playing on the first and second lines early
Edge stones enclose almost nothing and have very few liberties. The first line is called the line of defeat for a reason. In the opening, third and fourth lines only.
3. Playing solid contact moves
Touching an opponent's stone makes it stronger, because they get to respond and their stones gain shape. Attacking usually means playing one point away, not against.
4. Filling your own territory
Adding stones inside a group that is already alive achieves nothing. Under area scoring it does not cost points, but it wastes a move — and a wasted move in a close game is the game.
5. Answering every move
Not every move needs a reply. Before responding, ask what happens if you ignore it. If the answer is 'they gain three points' and there is a twelve-point move elsewhere, ignore it.
The tenuki habit
Playing elsewhere — tenuki — is a skill, not a rudeness. Strong players tenuki constantly. Practise deliberately ignoring one move per game and see what happens.
6. Ignoring weak groups
The opposite failure. A group without two eyes and without an escape route is a liability that will eventually cost far more than any territory gained elsewhere. Scan your own groups every few moves and ask which one is weakest.
7. Making empty triangles
Three stones in an L with the fourth point empty is the least efficient shape in go. It usually appears when connecting defensively. There is almost always a tiger's mouth or a diagonal move that does the same job better.
8. Starting ladders without reading them
A ladder that fails leaves a broken line of stones across the whole board and is often instantly losing. Read to the edge, or play a net.
9. Playing too close to your own strength
A wall of your own stones already controls the area beside it. Adding more stones nearby duplicates work. Extend away from your thickness, not toward it — the distance should be roughly proportional to the height of the wall.
Fixing them
The fastest way through all nine is short 9×9 games played quickly, then a single question after each: which of my groups was weakest, and what was the biggest point I failed to take? Two answers per game, six games an evening, and the instincts start to change within a week.