Freestyle Lines on Wrong Frames Basically the character is posed and rendered with various props. At random, suddenly, the lines are "sticking" among multiple frames despite the fact that the object has been moved. 

What could be the cause of this, and how can I fix it?

This only started happening recently - I had done many renders with this file / model and suddenly this has started. 

The magnifying glass frame is two or three frames down from the hammer one.
I can think of two things that you might have done:


You have Motion Blur switched on as suggested by Mechanic
You have used a transparent material to hide it.


I can't say anything else if you don't provide the blender file through https://blend-exchange.giantcowfilms.com/. With the actual file I would be able to troubleshoot your problem effectively cause I can't see your settings and stuff through the two images.