If that explanation were valid then the bug would be that you do not take damage falling into webs while riding a cart.
Either way, the behavior is not self consistent and suggests that there is a bug.
Further to my previous comment, falling damage is not reset by water or vines when in a minecart, but downward motion does get reduced to cobweb speeds when falling into a cobweb while riding in a minecart.
Is this expected behaviour or a bug?
I just ran a test for something that may be related. I arranged a minecart track to have the cart fall into a one block wide channel that had vines in it.
I can jump into the channel and take no damage no matter what height I jump from, but if I ride a cart into the channel I take damage as though there were no vines.
It seems that falling carts are not affected by things that reset your fall damage.
Is this expected behaviour or a bug?
Muscle memory will get you through this, until there's a problem logging in. Then of course you have no idea what the buttons are and have to guess. Yes this is a problem on the next pair of buttons too. Then it is extremely annoying.
Just to be completely clear.
I have observed this affecting the buttons with the labels [Options], [Login], [Try again], and [Play offline] in the launcher.
The reproduction steps under win7 are as follows:
Right click open area of desktop.
Select [Personalize] from the menu
Under "Change the visuals and sounds on your computer",
scroll down to "Basic and High Contrast Themes" and
select "Windows Classic"
Launch Minecraft