What I expected to happen was...:
The player should walk with "normal" speed after leaving a Minecart.

What actually happened was...:
The player keeps in "sprinting mode" (swimming is not effected), until you hit the "sprint key" again.

Steps to Reproduce:
Place a flat, straight railtrack in a Singleplayerworld (without powerrails).
Jump into a minecart, walk forward and hit the "sprint key" (CTRL).
Leave the cart regularly (sneak) and stand / walk / swim around.

After pressing CTRL again while moving, the player returns to "walking mode".