Submitted by: @pavel-zotov

Script:

show version;
set explain on;
select * from rdb$database where rdb$relation_id = 1;

Now connect to FB 2.5 but using ISQL of 3.0 and run this script.

STDOUT will be:

ISQL Version: WI-V3.0.0.32008 Firebird 3.0 Release Candidate 1
Server version:
Firebird/Windows/Intel/i386 (access method), version "WI-V2.5.5.26916 Firebird 2.5"
Firebird/Windows/Intel/i386 (remote server), version "WI-V2.5.5.26916 Firebird 2.5/tcp (balaha)/P12"
Firebird/Windows/Intel/i386 (remote interface), version "WI-V3.0.0.32008 Firebird 3.0 Release Candidate 1/tcp (balaha)/P12"
on disk structure version 11.2
Unknown error while retrieving plan

STDERR will be EMPTY.

IMHO, it will be better if STDERR would contain the same as when doing connect by ISQL 2.x: "Command error: set explain on". And STDOUT in such case also can be like 'old good 2.x':

Valid options are:
AUTOddl BLOBdisplay BAIL
BULK_INSERT COUNT ECHO
GENERATOR HEADING LIST
NAMES PLAN PLANONLY
ROWCOUNT STATs SQLDA_DISPLAY
SQL STATISTICS TRANSaction
TERMinator TIME WIDTH
WARNINGs WNG