Ranked by what a failure in that location actually costs you — not by how likely it is.
Exposed runs in a basement, utility room, or garage. Lowest consequence. Visible, accessible, and repairable quickly. Water lands on a floor you can see, and the repair needs no demolition. If you're going to have a leak, have it here. These runs are also where freeze risk is highest in unheated spaces, so low consequence doesn't mean low probability.
Crawlspaces. Low visible consequence, high discovery delay. Water goes into ground or onto a vapour barrier, and nobody looks. The pipe repair is usually straightforward once found; the problem is that "once found" can be months. The tell is a bill or a meter, not a stain.
Attics and pipes above upper floors. High consequence. Gravity does the rest. A failure here travels down through ceilings, insulation, and light fixtures, and it damages everything on the way. Attics also carry the highest freeze exposure of any interior location.
Inside interior walls. High consequence, delayed discovery. Water runs down inside the cavity, wetting framing and insulation before anything appears. By the time paint bubbles, the water has usually been there a while. Repair means opening the wall — to a point that has to be located rather than guessed.
Above finished ceilings. Very high consequence. Everything the wall case involves, plus a ceiling to open and reinstate, plus whatever was in the room below. The stain, when it appears, is frequently not beneath the failure — water travels along joists first.
Behind tile and in wet areas. Very high consequence. Because tile, stone, and specialty finishes generally can't be removed and reinstated. Matching existing material is often impossible, which means the repair is visible afterward regardless of how well it's done.
Under slabs. Highest consequence. No visible evidence, continuous loss under pressure, and erosion of the material supporting the concrete. Location is difficult, the repair options are limited, and delay reliably increases the cost. The tells are a warm floor patch, an unexplained bill, running water with everything off, and new cracking.