I can't get the ParallaxLayer mirroring property to work as it is documented.
Setting an offset to ParallaxLayer - Motion - Mirroring creates a second version of the child texture but the image is in the same orientation - not reflected. I found a workaround by manually adding another copy of the texture and flipping it with sprite offset. Also the API documentation for ParallaxLayer is worded in such a way it implies that if either Mirroring axis is set to zero no mirroring whatsoever will take place.
Godot V3.0.6.stable.official.8314054
Windows 10 Home V 1803 build 17134.345
Intel Pentium CPU G2030 @ 3.00GHz 3.00GHz
ASUS desktop with CM6730 motherboard with onboard GPU running Intel HD Graphics drivers
screenshot shows minimal tree to reproduce the issue.

Sounds like this is a documentation issue.
Mirroring in the case of the ParallaxLayer node means the distance over which the image is repeated ("mirrored"). Maybe it should be renamed to repetition instead.
Most helpful comment
Sounds like this is a documentation issue.
Mirroring in the case of the ParallaxLayer node means the distance over which the image is repeated ("mirrored"). Maybe it should be renamed to
repetitioninstead.