>We are running FoxPro 6.0 sp5 on Windows NT 4.0 network.
>
>In running a program, it came back with the error that VARTYPE.PRG could not be found. If a backup of this same program (located on a different drive) was run, it ran fine. (SAME foxpro session)
>
>To find out what caused the problem, I did a SET STEP ON in the problem program. When it hit the line in question, I checked the variables, and all was fine. I did RESUME, and the problem did not reoccur.
>
>I took the SET STEP ON out, and reran the same code, and no problem occurred.
>
>Apparently the debugger reset some things that "fixed" the problem. Any ideas on what caused the problem and what the debugger reset to "fix" the problem?

SET SWAG ON
It sounds like the PRG was compiled using a previous version of VFP (5 perhaps) when vartype() didn't exist. When you added the new line of code, it forced VFP to recompile the prg and the reference to vartype function was inserted correctly.
SET SWAG OFF

HTH.