A common thinking error is that people over generalise from just one act. 

If people break stay-at-home rules to see their friends/families, and don't get sick in days afterwards, then they are likely to feel an overly optimistic sense of safety and do it again and again.

Other common thinking errors is when people see cars coming and going from neighbours drives more often, workers undertaking work in peoples houses.

Once people observe a few people start to break stay-at-home rules, it starts to seem more socially acceptable to do these things

These signs of movement start getting problematic when everyone else starts doing them, and if everyone starts to think they aren't getting sick each time they begin to believe that they're safe, but in reality, things aren't as normal than they are.