<<Disclaimer: Verify this information before applying it to your situation.>>

My friends, I had an experience at my local McDonald's that I would like to
share to help you avoid a potential problem. I often get a vanilla
milkshake, but on this occasion, my shake tasted like strawberry. Although
I was a mile away, I returned and was told I had a vanilla shake, but the
pink indicated that there was some strawberry in the line from the previous
shake. Apparently, these lines don't always empty completely. At the time I
was not aware if strawberry shakes were GF or not, (they are!), but it
could just as easily have been chocolate--which isn't GF. Since then, I
always ask the servers to clear the lines and I watch to be sure I get the
right shakes. I guess I will give up on the drive-thrus...

I hope this tip helps ...

Aggie in Central NY