Whilst we try to make the directions as clear and unambiguous as we possibly can, there are times (especially when chatting) when everyone takes a wrong turn.
There are a number of things in the app you can keep an eye on to ensure that you're on the right path. The easiest way to check is via the map screen:
Another thing to watch out for on the summary screen is if the distance to the next direction is INCREASING!!
... 
Where it can, the app will attempt to warn you if you are wandering off-route:
However there are certain directions where it won't do this when a legitimate divergence from the route is anticipated as "likely", for example to explore the beach (where being nagged would be annoying):
A quick glance at the map or the distance to the next direction is the guaranteed way to avoid any long backtracks.