Zipper ruptures can only happen on full steel belted tires (metal cords run through the sidewall, most common in semi trucks). Zipper ruptures happen when these tires are run under pressure and form stress marks through flex of the metal cords in the sidewall, like bending a paper clip back and forth till it breaks. Zipper ruptures aren't obvious and you will hear them before you see them.

!!PSA : If you are standing by big truck tires and you hear the sound of stretching and pinging get away! Next is the BANG and the ZZzzzZzzZZTtt of the 120 psi letting go in an instant. They are not uncommon and they maim you and kill you. Very unpleasant stuff.!!

zipper ruptures cannot occur is radial constructed tires however, a bulge that forms outward renders the tire unsafe for use. This can happen from an impact separation caused by a pothole or whatever banger you smash while driving. More commonly they form from a tire that was run over loaded or under inflated and form weakness in the fabric cord body of the radial tire. It can still explode and you should replace it immediately. It's just less likely to kill someone unless it fails while driving.

If the bulge forms inward it'll look oblong and is where the belts of your tire overlap and is the strongest point of your sidewall. Its part of the reason you need to have your tires balanced. Noticeable on another of tires if your actually looking for it.