Check out these two contrasting maps of the Ballachulish Horseshoe - notice anything missing? One example (it should be obvious which!) is the Ben Nevis & Glencoe British Mountain Map downloaded onto Viewranger. This isn't cheap - £24.99 for the download, but - here's the clue - it has a very clear symbol for cliffs. The other example is from the free version of the Great Britain Topo Maps app, which is currently the top-rated GPS app in the Google Play shop.
This is a genuine example which came about as I was working with a client on the classic scramble up Schoolhouse Ridge above Ballachulish - highly recommended for a nice introductory scrambling day. He was keen to show me his excellent free GPS app. When we started to navigate down the blunt spur running South East from the summit of Sgurr Bhan 947m, the limitations of free open-source mapping soon became apparent!