On the way to a music festival early in the morning I took a wrong turn due to confusing instructions from google maps and ended up in Bristols clean air zone for a few minuntes while my sat nav rerouted me.
I had no idea because I didn't see any signs (although since looking on google streetview it was very well signposted but I wasn't paying attention to them because I made sure my route didn't go through it before I set off)
I have since been sent a PCN for £120 + the £9 charge; reduced to £69 total if paid within two weeks.
I would've paid the charge if I'd known about it but £120 on top seems excessive for a simple mistake.