Photoshop question - what is this "artifact?"

[valdore]

See attached --

Often times when I'm Clone Stamping, it seems if I get too near an object in the photo, it will then be duplicated as I stamp elsewhere?

Why is this, what am I doing wrong if anything, and how do I prevent it?

[stagi]

You are using too large of a brush and the hardness of the brush is set too low, so you think you aren't on the object but the brush actually is so it is cloning the object you don't want.

[valdore]

Ah... thank you.