The single biggest mistake: painting a small swatch directly on one wall and deciding from it. The surrounding wall color contaminates your perception, and you can't move it to compare against different light.
Better method: paint two coats on a large piece of foam board or poster board with the same finish you'll use on the wall. Move it around the room throughout the day — corners, near windows, far from windows, and at night under your actual lamps.
Peel-and-stick samples from Samplize or directly from Benjamin Moore are excellent and faster than wet samples — just confirm the print accuracy if the color is a critical match.