In the animated GIF I had used the Tape Measure tool a great deal to make various solid lines.
I was trying to make a vertical line 4" to the left of a previous Tape Measure line. When I tried to do so, the beginning point of the dotted line suddenly changed from red to green. I got a horizontal line instead of a vertical line.
When your Group axes do not align with the model axes inferencing is a bit more difficult to use. When the Group/Component axes are aligned with the model axes, red, green and blue are generally what you’re looking to see. If the Group axes are different, you may need to look for the pink inferencing lines.
That does not really answer my question. I want to know why my guidelines suddenly start going 90 degrees away from what they previously did. Is this a SketchUp bug?
It’s not a bug, nor is it green, it’s a blue diamond, indicating that your tape measure is on face, or inferencing a plane. The green circle is different, and is the indicator for being on an endpoint, Red square indicates on edge. Without having access to your model I can only guess that the area behind the guidelines you show is a plane, a surface, and that your tape measure is in fact inferencing that plane rather than the guide you intended.