Shutter speed changes automatically for different shots?

I'm currently shooting a short with a Nikon D3100. I shoot wide open with a shutter of 1/50 and have the camera set to manual mode for obvious reasons. I'm now looking back at the previous scenes I've shot and I noticed that some shots have motion blur (which is expected from 1/50 shutter) and other shots in the same exact scene have absolutely no motion blur at all. I'm looking at a person move their hand and there is motion blur followed by another shot where someone is moving their hand with no motion blur AT ALL. It looks like I shot it with completely different settings which makes no sense at all because I ALWAYS have it on manual with 1/50 and f1.8. Is this a common problem? Is there a factor I'm not taking into account? Is the lighting screwing with me? Is the camera mentally slow? I'm so confused right now and I was hoping someone might have a clue.
Added (1). It looks like as if I'm shooting 1/250 shutter when it clearly says 1/50.

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