I've been aware of this potential problem for some time, and looked out for it carefully, but it's never actually happened - although I can see how it could. I generally use quite skinny ropes, but the devices I use (or have used) have minimum diameter recommendations well below these.
In the past I've used BD ATC Guide, and WC Pro Guide Lite, both of which are suitable for ropes down to 7.7mm according to the manufacturers. Currently, my main device is a Giga Jul which is ok down to 7.1mm, apparently. My skinniest doubles are Beal Ice Lines 8.1mm, skinniest single is Edelrid Swift Protect 8.9mm. I will shortly be on a trip climbing sport multi pitch, using the Swift as the main rope, pairing with one Ice Line for abseils.
You would think that if manufacturers rate their devices for a certain range of diameters, this would include all normal usages, including guide mode, and there should be no possibility of the ropes squeezing past one another in the slots; however, I've heard reports of this problem occurring with ropes considerably thicker than the official minimums (which are very skinny indeed).
Any experiences/thoughts? Thanks.