Understanding conventions that differ from what you're used to.
Distances are shown in kilometers in most countries outside the US, and elevation and terrain symbols can vary meaningfully by region. Highway numbering systems and color-coding for road types differ by country, so a quick look at the map legend before you set off pays off.
Local place names sometimes differ from what's shown on English-language apps, particularly in regions with multiple official languages. Cross-referencing your GPS with printed signage as you go helps catch this early.