As suggested by Emma, check for not standard Inchi's:
#Find not standard inchi's + corresponding inchikey
SELECT DISTINCT ?item ?inchi ?inchikey
WHERE
{
?item wdt:P234 ?inchi.
FILTER(STRSTARTS(?inchi, '1/')) . #Not standard inchis start with '1/' (standard with '1s/').
OPTIONAL {?item wdt:P235 ?inchikey}. #Not standard inchikeys end with NA-X (standard with SA-X).
}
As suggested by Emma, check for not standard Inchi's: