I'd say the fact that these bugs only very occasionally happen - relative to the huge number of shell scripts out there that are being executed every day - that it's not really "frighteningly common". You only hear about the ones that fail.

By the same logic, memory safety issues only happen rarely, right? Most programs/scripts are going to be tested if part of a distribution and such errors removed. But without polling people it'd be hard to know of the many times this kinda thing messed things up. I personally wiped out a production DB due to expanding an unset variable (fortunately immediately after taking a backup.