Strictly speaking it's usually the frame that doesn't meet spec, not the window/glass itself (tho sometimes you can see failed IGU's from the condensation inside the two panes, or discolouration when seen from outside). The failure is usually as it pertains to a leak test, and could be because of improper sealing in the shop, lousy design, improper gaskets, improper installation procedure, or sometimes even lousy fabrication (joints that don't meet evenly, blocked drainage channels, or screws through an air or wet seal, that sort of thing)
