This is Bob. Bob is conducting an observation on flowers, and how they grow differently with different fertilizers.
He goes out and checks on the flowers taking down notes everyday on what he sees, any changes that may have happened to the flowers overnight
One day he has to work overtime at work, and misses the time for checking the flowers, so he went at night, while he was tired thinking it would not change anything in his observations
When he went to check that night, he wasn't as observant, due to exhaustion, so he missed that because of one of the fertilizers that he used, one of the flowers changed colors at night, due to the temperature. This caused him to have an inaccurate observation in his experiment.