This is usually caused by flickering artificial light rather than a fault with the camera. Fluorescent lights and many LED lights don’t emit perfectly constant light; they pulse at the mains frequency or via rapid on/off dimming. If your camera captures the scene during those pulses, you can get dark horizontal bands.

The effect is often worse in low light and with dimmed LEDs, because the light spends more time “off.” Using the electronic shutter can also make the problem more visible, since it scans the sensor in a way that can interact with the light’s flicker.

To reduce it, try:

• using the mechanical shutter instead of electronic shutter
• avoiding dimmed LED or fluorescent lighting when possible
• changing shutter speed to better match local power frequency (often around 1/50 or 1/60 and related speeds)

This is a common issue in stage and event photography under LED lighting, not something specific to the X-T1 alone.