Larry, running some tests on my side (but not with debugger, and not being overly familiar with the SQLite internals), it's real hard to find other ways to cause this, but I did come to believe the problem originates in the Window function return rather than the group_concat() code itself. This deduced by manner of poking the black box with a stick and seeing how it rolls, so I could be very wrong, but posted on the off chance it saves some bug-hunting time.