Going too low on soft steel
A 15-degree edge looks sharp right away on a cheap chisel, but it rolls over in seconds because the steel is too soft to hold that thin an edge. Bump up to 28-30 degrees and the edge will last longer even though it feels less razor-sharp at first.
Ignoring the micro-bevel
You do not need to re-grind the whole 25-degree face every time. Just add a tiny secondary bevel at 28-30 degrees on the last few millimeters of the edge. It takes 10 seconds on a stone and gives you a much tougher edge for hard use.
Sharpening at one angle for every tool
A paring chisel and a mortise chisel do very different jobs. The paring chisel can run at 25 degrees for easy slicing. The mortise chisel needs 30 degrees or more because it takes heavy hits. Use the table above to set each tool for its actual job.
Not checking your jig setup
If you use a guided sharpening system like a Tormek, Edge Pro, or Worksharp, double-check the angle setting against a digital angle gauge. Jig registration can be off by 1-2 degrees, which makes a real difference on kitchen knives.
Skipping grits
Jumping from a 400-grit stone to a strop leaves a rough edge that dulls fast. Work through at least three grits: start around 400-800 for shaping, move to 1000-3000 for refining, then finish at 4000-8000 or on a strop with compound.
Forgetting to hone between full sharpenings
You should not re-grind every time the edge feels dull. A few strokes on a fine stone or a leather strop brings the edge back without removing metal. Save the coarse stone for when the edge is nicked or the angle has changed.