Let's hope common sense prevails. What would be achieved by prosecuting? There must be other more important issues to allocate resources to first.
If a ticket should be retained by the barrier or marked as consumed by the barrier then avoiding that would be evasion. In that scenario when challenged prosecution should be encouraged.
If this does not apply and there is no harm caused the why prosecute? If someone is detained unnecessarily and their time wasted because of cost cutting on staff or unfit equipment then that is a material harm to the customer. In general contracts supplier to customer a reasonableness test would apply.
I can understand that having to challenge people unecessarily is a waste of resource but one presumes the challenge to success rate justifies the challenges.
Profit > Common Sense every time.