Title pretty much says it.  As an example, I would expect this code:

G91 G18
G02 X-2 I-1

to begin movement in the -Z direction, bottom out at Z-1, X-1, and then start moving in the +Z direction finishing at Z0, X-2. In other words, clockwise circular interpolation. To get that behaviour I have to instead use G03.  In G17 mode this isn't a problem - G02 is clockwise, G03 is counter-clockwise, as expected.

I'm using Version R3.042.029 (the current Tormach distro)

It's not really a big deal to work around, but I rather dislike having to be cognizant of it all the time - I'm using it a lot right now for the work I'm doing.

Make a fist with your left hand and your thumb sticking out. In G17, point your thumb Z+, and your fingers are pointing clockwise.

For G18, point your left thumb in the Y+ direction. Your fingers are pointing in the clockwise direction. Opposite of what you're expecting. And correct.

Got it.  When I'm standing where the operator normally stands, I'm looking at the XZ plane from the negative end of the Y axis, unlike the XY plane, which I look down upon from the positive end of the Z axis.  Likewise, if I arbitrarily stepped around to the left end of the table (X-) to look at the YZ plane (G19) it would also seem counter-intuitive, but if I were to step around the right side of the table (X+), it would behave more like I expect.

Good lesson.  Thanks.