All too often I'm handed a map to create for a client with some pretty common mistakes. If you committed any of the following, don't worry! Not only are you in good company but you also shouldn't expect to know everything! Especially regarding things you won't even think to research!
The following list is ranked most to least common mistakes most people make on their maps:
Rivers run from higher elevation and end in at the coast into the ocean. They will start in mountains or hills and run to lower regions where they might connect with larger rivers but they almost all flow into the ocean.
Rivers join, or flow into, other rivers but rarely split. This is because the river already has a path and if there is suddenly more water, that path just widens, not suddenly split to alleviate the heavier flow.
Rivers do not split from one river to join another.
There is a "wet" and "dry" side to a mountain range. The wet side is where the majority of precipitation is released onto the land. The dry side is dry because it gets less rain. This doesn't mean, however, that no rain falls on one side or that a desert immediately follows a mountain range. It also doesn't mean rivers only flows from one side of the range and not the other.
A desert is a landscape that has little to no vegetation. This is why we distinguish an "arid" desert from a frozen one. A desert doesn't mean it's hot.
A frozen tundra can be right next to an arid desert. Though there would likely be a stretch of land where the two biomes mingle for a bit, it is still possible since most arid deserts experience extreme temperature changes from day to night.
The most difficult terrain to traverse is wetlands, not mountains. Though mountains might make for an arduous journey, it is much easier to hike across a mountain than it is to slog your way through a wetland. Especially with any vehicles or heavy equipment.
All your maps are in the past. They were created by someone easily a year or more ago by a cartographer interpreting data from different sources. This is why we update maps regularly to account for new roads, towns, changing coastlines, etc.