I just had a strange error involving the assistant tool in the release candidate:
I sketched on the canvas (7 layers) and then placed two vanishing points. Unhappy with their placement, I attempted to move them by grabbing their little dotted feet and then their nifty new label, but nothing worked. Placing new vanishing points no longer worked, either. Curious, I took out the infinite ruler assistant and tried to place it in many places - but the only place it worked was on the lower left hand quadrant (pretty far from the two vanishing points). I managed to place another there, but I still could not place more in the upper right  quadrant. It was the same with the ellipse assistant, until I placed the first part in the lower part and the next point in the upper portion. After that, the assistants seemed to reset and now I have a big mess of them on my screen and can move them. Drawing on the screen is not creating any noticeable errors, but somehow during this my canvas has doubled in size to the right! I recently posted another error regarding assistants at https://bugs.kde.org/show_bug.cgi?id=363385 , so I am assuming they are related, but I don't know what's going on.

Good luck!

Reproducible: Didn't try