The fact that they were doing something different is a potential insight or not part of the overall study. It is up to your judgement whether or not your data is valid and adds value to the study.

• You want to observe people as though they were doing every day tasks.
• You do not want to tell them they are wrong or what they are doing wasn’t right.
• You also don’t want to explicitly tell them what you want them to do.
• Instead have them find that out on their own to understand their behavior.