One big mistake is ignoring the length. Boxy tees are shorter by design, so if you're tall, the hem might sit higher than you're used to. Check the length measurement on the size chart and compare it to a tee you already own that fits well.

Another mistake is assuming your usual size is the same across all styles. It's not. A boxy cut is fundamentally different from a regular or slim fit. Always measure and check the chart.