To reproduce:

• Place a Minecart rail that terminates in front of a solid block A (a common booster mechanism when the end is powered rail). Place a solid block B beside the rail end, leaving one space in between. Put a button on B, facing the rail.

• Enter the minecart. The player will not fully enter the cart (as seen in third-person view, the player hovers slightly above the cart), though considered to be sitting in it. Due to this, it will be impossible to press the placed button - instead of activating it, the click will cause the player to exit the cart.

• Break block A. The minecart can now be entered without difficulty, and the button can be pressed while inside.

This has occasionally occurred in 1.4.* because a cart could slide part-way into block A, but that could be fixed by breaking and replacing the cart. Since 1.5.0, it has affected recently placed carts as well. I'm not sure what causes this, but it feels like a glitchy collision check.

(A sample bi-directional train stop that suffers from this regression is in the screenshot.)

Note that this can be worked around by moving all buttons one block higher, but they will then no longer power the rail directly, needing more wiring.