It is a curtain issue (shutter). Usually you will see this when using a flash and trying to use a shutter speed faster than the sync speed. If this is happening in normal operation (without the flash) I suggest contacting a service department.
Yep. The above post nailed it. You can't exceed your sync speed. This is often like 1/250 of a second. So you'd have to use like high speed sync on the flash. Otherwise, the flash itself often has enough stopping power, that you don't need a super fast shutter.
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