Do 'writing errors' show up when burning dvds?

I am reading lots of reviews, and keep seeing that almost all dvd writing drives have PI errors (whatever that is) when writing.... what does this mean?

Does it show up when you are watching the movie? Does it affect the quality? I thought copying a dvd (with no compression) meant it was a 100% exact duplicate?

Can someone jump in and educate me a bit on this?...lol

Just waiting for the answer on this and then I think I am going to jump on a LG 4163, it's getting really good reviews for writing on single and DL.

Thanks!