Wow @nathanmarz, I came here looking for exactly this answer.
(select-one ALL [:a :b])
Execution error (IllegalArgumentException) at com.rpl.specter.impl/compiled-select-one*$result-fn (impl.cljc:375).
More than one element found in structure: :b
(select-one ALL [:a nil])
Execution error (IllegalArgumentException) at com.rpl.specter.impl/compiled-select-one*$result-fn (impl.cljc:375).
More than one element found in structure:
https://github.com/nathanmarz/specter/blob/7790213b163b07ef830fdbb1745fffcda61d40f6/src/clj/com/rpl/specter/impl.cljc#L379
Here should be (pr-str structure)
to better error messages.