I wonder if it will be a good idea for each country or travel type, to have specific thread for gotchas. These are things you don't normally find out until you are there at your destination.
For example, travellers from Canada or the US may find Australia or Denmark to be really expensive.
A hamburger in Australia will start at 14AUD.
A ticket to snorkel at the Great Barrier Reef does not include the 7AUD recommended lycra suit.
In Denmark, asking for a glass of water with your meal at a restaurant may cost 20Kr.
Tour guides in Russia collect 5USD to let you take photos. That or I just got ripped-off.
Russian locals would rather take Euros than Rubles.
You can't exchange your currency in China unless your bills are as good as new (no rips, folds or scribbles). But they'll give you RMBs that could be in tatters.
In the Caribbean - Diamonds International.