I had rear struts put on a while ago. Every time I went over a bump, the
rear end would lift and shift, we have some bad roads here in Ottawa.
Anyhow, after a week of experiencing something similar, I took the car back
and it turned out that the Nissan Struts were defective and had to be
replaced again.

So, I'd say it's not normal to get defective parts off the shelf, but it
does seem to be possible.

rtt