Pulling your paddle grip inside of your canoe gunwale, which puts your blade at an angle when ruddering. Instead, extend your upper hand out so the grip is outside the gunwale.
Not heeling the canoe's hull into the direction of a turn.
Trying to draw or sweep the bow against the current to change the canoe's direction instead of prying or drawing the stern downstream.
Changing paddling sides to steer the canoe.
Not holding the paddle vertical when doing a power stroke.
When solo paddling or paddling stern in a tandem, not doing a rudder at the end of the power stroke.
Leaving eddies with too much angle when starting an upstream ferry.
Not starting a stern pry with the paddle blade touching the side of the canoe at the stern, and prying the paddle until it is so far out that it kills forward momentum. Inappropriate reverse sweeps create the same problem.
Not burying the blade completely under the surface when executing strokes.
Not enough forward momentum when punching eddy lines.
Using a paddle that is too short.