Take a look at the GIF.
I need to measure the width of one square, and then measure the overall width of both squares.

I create the first dimension with no problem.
However, when I try to create the second one, Ai plays dumb.

When I click the leftmost point, the tool thinks that I am to EDIT the existing dimension (although I am clicking and not click-and-dragging, and I am clicking an endpoint of it, and not in the middle!)

Well... I can lock the first dimension (which would require me to abort, select the first one, lock, and try again).
Instead, I try to start from the rightmost point and click the leftmost one then.

...but Ai now plays even dumber, it doesn’t allow me to FINISH my dimension! It SELECT the existing one instead, although I am in THE MIDDLE of CREATING a new one!

OK, I trick it and snap to the point at the bottom — but then I get a skewed dimension instead of a parallel line, and there is no Shift I can quickly tap to force it to be orthogonal, and now I have to draaaag over to the right to make it behave — which is a separate request I made here:

'Sure', you might say, 'it’s your own fault, you made the offset for extension line to be zero!'
Yes, I did. I need it to be zero. Our standards require it. And yours seemingly permits selecting other objects while in the middle of creating operations....

Curvature tool has the same exact problem, reported here: http://illustrator.uservoice.com/forums/333657/suggestions/34578331

Get your shit together, Adobe.