MBP's trackpad button is wearing out. Not sure how to describe it, so can't search if it's common or not. Like all mouse buttons, it clicks when you press and clicks again when you release. However, when dragging, it tends to release in the software before it clicks. This can be avoided by pressing harder, so I think it's wearing out. Problem also appears when using a nonconductive object to click, showing it's not a static problem.
I wonder how much it costs to fix? And why do Apple's buttons wear out so easily?
Update 12 Feb 12
Now the problem is gone. Maybe it can be fixed with a restart?