>>I just tried adding an ON ERROR STRTOFILE() immediately before the REQUERY(): the behavior stopped occuring. When
>>I commented it out again, I saw the behavior return.
>
>Bingo. Any wait state or other attempt to debug it will make the code work. If you compile a sample into an EXE,
>you will get reproducable result almost all of the time. As you enter WAIT states or function calls (which trigger
>events and thus wait states), you get more variability.

Do you have an error handler in effect in your program? If not, maybe adding one is a quicker workaround than recoding everything in VFP6.