Sometimes parent page missed nodes pointed at lower level pages.
For example parent page have nodes A, B while at lower level pages linked as A, C, B
This error can be detected by gfix. It reports in firebird.log 2 errors for the same index page for each such case :
Index XXX is corrupt on page YYY level 1. File: \Fb2\fb2.0\src\jrd\validation.cpp, line: 1656
Index XXX is corrupt on page YYY level 1. File: \Fb2\fb2.0\src\jrd\validation.cpp, line: 1646
(line number as per 2.0 sources)
This error itself can't lead to wrong query results, AFAIK, but i don't know if it can lead to more serious corruptions being present in actively modified index for a long time.
Also this bug can produce 'wrong page type expected 7 found XXX' bugchecks