[r85804306610]

you probably didn't define the function, f(x,y), which is the line before plot3d. it's a slightly buggy behavior

[michaelsbradley]

Ah, yes, can reproduce the problem if f(x,y) isn't defined.

Also, if f(x,y) is "broken" (e.g. by a typo like syn instead of sin) then the effect is the same.