A good example is Australian low-cost airline jetstar.com:
To start off with, the site works pretty much as you'd expect. You do a search and pick the flights you want. Then you get to the seats page, below:
What's deceptive is the way the page above presents your options: it uses misdirection to hide what is actually happening here. When this page loads, they have already preselected a seat for you, at an extra cost of $9 ($4.50 each way). This is deceptive for two reasons. Firstly you don't get to opt in - they have effectively sneaked it into your basket without you asking for it. Secondly, they're charging you extra to select a seat yourself, but you haven't actually chosen one, so you're effectively paying for nothing. If you're not concentrating, you might end up proceeding to the next page having paid $9.
Above you can see more of the body of the page, which is entirely devoted to seat selection, heavily promoting higher priced upgrades ("extra leg room" or "up front"). If you're really concentrating, you'll scroll all the way down to the bottom of the page without clicking any of that stuff, and you'll eventually get to the small text at the very bottom.
Above you can see a tiny link below the "CONTINUE" button that reads "skip seat selection". That's what you have to click in order to avoid paying for the unwanted seat upgrade.
According to the Jetstar site, they run over 4000 flights per week - which works out as 208,000 flights per year. If every customer is caught out by this trick ($4.50 per flight), this accounts for almost $1 million of profit each year.