Web, yeah, they're common. Unfortunately, it's people that put the stuff down, and people are imperfect. We try our best to give a perfectly flat surface, or at least no glaring ridges, humps or dips, but it all comes down to 'how good a trowler are you and are you having a good day at it'. There are a large number of variables involved but, the biggest one is how much the company doing the work cares about it's finished product. We carry about 1/2 mix when we go out to drop the liner, to correct any visible imperfections in the floor (and patch the holes where we staked in rebar to put up the strings which define the walls and floor). We sweep the bottom 3x to insure that nothing, but the floor, is left behind under the liner, and check for ridges and 'low spots' that might need attention, still, a light in the pool will reveal some imperfections that we couldn't see (at least you're getting a 'custom' bottom - no one else's will be exactly like yours ) It's inherent in the process, unfortunately, but is not a 'defect' in your pool and will not harm the liner. If you'd like more info, just ask