Liable to error, bail fell out! (8)
I believe the answer is:
fallible
'liable to error' is the definition.
(thesaurus)
'bail fell out' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'bail'+'fell'='bailfell'
'bailfell' with letters rearranged gives 'FALLIBLE'.
(Other definitions for fallible that I've seen before include "Capable of erring" , "Northern Scandinavian" , "Likely to fail or make errors" , "errant, possibly" , "Capable of making mistakes" .)