One of the most unusual leaks I've heard about was in a unit with dual evaporators. There was a leak where the liquid line branched to two TXVs. (Visualize a single liquid line on one end of the fitting and two smaller liquid lines side by side on the other.) Apparently, the brazing was marginal so it cracked between the two smaller lines.

Strange thing is, ever since I heard about it, I somehow get reminded about it every month!