Common mistakes
A move is wrong when the digit you place already exists in that row, that column, or that box. Killer adds one more failure: the cage repeats a digit, or the cage no longer can reach its printed total. The game can flag the wrong cell. The fix is still to remove that digit, not to force the rest of the board around it.
Repeating a digit you just looked past
The usual miss is a digit at the far end of a row, or a digit in the same box but not in the same row as the cell you are filling. Before you commit, glance at all three: across, down, and around the box outline. Given clues count. They are easy to skip because you did not type them.
Treating a note as the answer
Notes are candidates. If a cell shows 2, 4, and 7, none of those is placed yet. Filling the cell with 4 while 4 is still possible in another cell of the same row is a guess. Guesses are what the three-mistake limit punishes. If Settings has Limit Mistakes to Three turned on, the third wrong entry ends the puzzle. Turn the limit off if you want to keep going after a wrong digit. Undo steps back either way.
Leaving a note that can no longer be true
You place 6, and a note of 6 remains in the same column. The next time you trust that note, you repeat the 6. Auto-Clear Notes in Settings removes those for you when you place a digit. If it is off, erase the stale note yourself. A hidden single is also easy to miss while those extra notes are still written down. The singles guide shows that case.
When you are actually stuck
Hint fills one correct cell and leaves the rest. That is a better recovery than placing a second candidate “to see what happens,” especially with the mistake limit on. If the board is too far gone, Restart Puzzle on a finished or failed game starts that same puzzle again without hunting for a new one.