Apparently if they cut too close, the tip of the hair can go back inside your face, then when it tries to grow out, it misses the hole and grows under your skin. Some guy on another forum said his dad had this solid lump on his face for about a year, getting gradually bigger, then one day he attacked it with a needle and about 15" of coiled up hair came out. (Possibly a) True story.
Anyway, another vote for mach 3.