I got a notification from NHS track&trace to isolate due to contact I'd had. Doing the maths that was a Saturday when the only place I'd been was to a shopping centre to pick up groceries and a plunger.
Fortunately I tested and was negative, but could easily have been a positive. I was following all of the rules, going out for essential items (still Tier 2 at this point btw), wearing a mask, and distancing. I live alone btw, so that is the only social contact I'd had.
Not everyone getting the virus is 'breaking the rules', in fact I'd hazard a guess that most aren't.