Without proper instruction, voters may not know how to mark such ballots correctly. For example, a voter may draw an "X" between the two arrow halves, draw a circle around one or both halves, connect the two halves with multiple lines, or unknowingly use the wrong type of writing implement. Alternatively, a voter may know how to connect the arrows correctly, but may attempt to correct a mistaken selection by crossing out, scribbling over, or making notations. When errors of this type prevent votes from being counted, the ballot is described as "spoiled."
Even when a voter understands very well how a ballot should be marked, there still can be mistakes and accidents that spoil a ballot. For example, a ballot may become spoiled by stray marks, smudges, stains, loss of control of a pen, or accidental use of pen that has the wrong color of ink.
When a voter spoils a ballot without recognizing that or without being able to correct that, the integrity of the election is reduced. When the voter recognizes a spoiled ballot and is able to take corrective action, this still may degrade the election experience for that voter or for other voters.
While election integrity is the higher priority, the quality of the election experience is important as well. Each of the use cases described above provides an opportunity to consider the election process and ways in which it might be improved.