Elf Witch said:a city guard with a crossbow is a threat no matter how high level you get.
How about chain criticals? As in 20, 19, 20, 19 with a light crossbow would be 1d8, x2, x2, x2. 6d8 or 8d8, either way, it's a very rare possibility, but possibly important to a PC. Probably ought to require a 20 to chain, though . . .