Obscure question on rendering single frames

[Dan Ebberts]

When you have a comp open and do Composition > Save Frame As > File..., most of the time your frame goes directly into the render queue, but under some conditions you get an intermediate "Output Frame To:" dialog where you can navigate to a folder and/or change the output file name. Apparently one condition that will cause this to happen is if you've never previously rendered anything with that version of AE (and when you put something in the render queue you get "Not yet specified" for the "Output To:" file name).

But I suspect that's not the only condition that will cause the "Output Frame To:" dialog to automatically pop up when you try to do a Save Frame As. Can anyone shed any light on this?

I'm asking about this because I've written a script for a client that uses a menu command to invoke the Save Frame As function, but it fails when that dialog pops up.