For example, some people immediately leave negative score when they receive the DOA product or when the product fails during the warranty period - I simply ignore such reviews (any product has a chance of being defective) unless there's a significant amount of such reviews.
On the flip side of that there can be products with good user reviews that will eventually prove to be unreliable, simply because the product is too new to have had many failures yet.
I bought a Viewsonic monitor on newegg a few years back and when I bought it there were almost no negative reviews (the monitor had only been released a couple months prior). After less than a year that monitor spit out some smoke and died, and I went back to check out the newegg reviews for it. By that point the monitor had racked up a large amount of 1-egg ratings due to failures exactly like the one I had experienced.