Even though it's a surprise and it's expensive, at least the Denver collectors aren't jerks. I dropped my wife and son off at the Denver airport years ago and didn't give the toll roads much of a thought (no toll booth? huh). A few weeks later I got a nice letter with instructions to pay the toll. Fair enough. We've done it again a time or two since then and it was all very civilized.
Fast forward to earlier this week. I got a "NOTICE OF TOLL EVASION" letter after my wife drove on a toll road down near San Diego (I'm not sure where the toll road actually was). The letter says that the toll is $6.49 and the fine is another $57.50. Since they're super swell people, they're waiving the fine this time, but they say that there won't be any grace for the fine if she does it again in the future.