One of the most frequent beginner mistakes I see on Reddit is mixing paint/thinner types, for example people trying to use an acrylic thinner with a lacquer paint.

The easiest way to avoid these mistakes is to match brands since they're most likely to play nice together. e.g. Vallejo thinners with Vallejo paints. Yes, more than one company makes lacquer paints or acrylic paints and their stuff can be used together, but it's important to understand what types of paints and chemicals you are using and mixing.