You undercook or overcook your burgers. Consider
this your reminder to judge your burger’s doneness by the thermometer, not the
color. Ground beef should be cooked to 160 degrees.
You overseason your beef. Underseasoning or overseasoning ground beef can lead to an unpleasant fork full of flavor. Start conservative, and you can always add more if needed.
You are cleaning your cast iron skillet incorrectly. A little dish soap and scrubbing brush to clean up your cast iron skillet, and you should be good to go, right?! WRONG. You should wash cast iron by hand with water while still warm, and then dry promptly.
Don't overpat your burgers. We get it. It can be tempting to pretend you work at Smashburger, but really, there's no need to smash your burgers - they will cook evenly and keep their tenderness without the extra love!
You use the same knife on your raw meat and vegetables. You should NOT use the same knife for raw meat and raw veggies. Cross-contamination can easily occur if you are not careful, so it's best to keep raw meat and raw vegetables on separate cutting boards and use separate knives.
You aren't slicing against the grain. Always slice AGAINST the grain for the best beef eating experience.