In older versions of Minecraft (I believe those before powered rails were introduced), minecarts and activated TNT, if in a water stream, would be pushed (somewhat slowly) by the water; this no longer occurs.

Whether or not this was disabled deliberately, I request that it be re-added, because it adds more interactions to the game and enables interesting mechanisms:

• Minecarts:
  • Separating stuck-together minecarts, a glitch which can occur in complex automated minecart systems.
  • Extremely slow timer mechanisms (similar to items falling through cobweb, but without destroying items).
  • Horizontal cart storage queues.
• TNT:
  • High-power or compact cannons using water to push multiple TNT into one space.