Every wrong entry adds 10 seconds to your Effective Time — the number that actually decides a race, not the clock on the wall. Hit three mistakes and the race is over immediately, whatever time is left. So these aren't vague "bad habits." Each one has a price.
1. Guessing when you're stuck
You've stared at a cell for ten seconds, two digits still feel possible, and the clock is running. Placing the one that "feels right" is a coin flip you're paying 10 seconds to lose half the time. If a cell has more than one real candidate, it isn't solvable yet — move on and come back once something else has narrowed it down.
2. Reading the wrong box
Easy to do at speed: you check a cell's row and column, miss that its 3×3 box already has the digit you're about to place. This is the single most common way a "confident" fill turns into a mistake. Worth the half-second it costs to actually glance at all three before you commit.
3. Trusting a candidate you never re-checked
You noted three candidates for a cell early on, solved four more cells since, and never revisited it. One of your own placements probably killed a candidate you're still treating as live. Candidates go stale the moment the board changes around them.
4. Racing after your first mistake instead of resetting your pace
The instinct after mistake one is to go faster to make up the lost time. That's exactly backwards — you're now one mistake from a forfeit, and rushing is how you get there. Slow down by a beat, not the other way around.
5. Skipping the box you've been avoiding
Every board has one region that's messier than the rest — more candidates, less obvious. Solvers tend to keep circling the easy boxes instead, which just delays the mistake rather than avoiding it, since the messy box has to get solved eventually, usually under more time pressure than if you'd tackled it first.
None of these are exotic. They're the five ways a raw solve turns into a slower Effective Time — worth knowing before your next race, not after.