Nintendo Switch controllers can suffer from thumbstick drift. All gamepads are vulnerable to this flaw, but it seem Joy-Con are particularly susceptible. That means it can pay to keep your console in good condition, investing in a case that leaves ample room for the thumbsticks to remain upright. However, even with all the love and attention in the world, those pesky sticks can still start running away from you.
If this happens it's worth first recalibrating the controls in the System Settings (Controllers and Sensors in the menu). If that doesn't help, it's time to get into the nitty gritty and clear out those sticks. You'll need a can of compressed air and a small screwdriver. You'll notice a small rubber flap at the base of your thumbstick, lift this gently with the screwdriver and carefully spray the area underneath with compressed air. This will dislodge any dirt or dust that's clogging the underside of your controller.
If all else fails, it's worth noting that Nintendo is operating a Joy-Con drift repair scheme free of charge.