Description:
This was something I noticed while playing with the grenade launcher and doing bank-shots down the first floor entrance shafts. Basically, you can get a grenade "stuck" in a 90 degree corner by aiming at close to a 45 degree angle directly into the middle of the corner. Success seems based mainly upon angle and having a bit of distance; zoom helps as well.

When done successfully, the grenade is stuck in the corner and instead of falling, climbs upward until it reaches the top of the object (the tower in this case). If it reaches a flat surface, it will come to rest and bounce a bit in place. If it reaches an angled surface (the forward launch shafts of the base, for example) it will roll off as expected.

On first look, this doesn't appear too useful, but could potentially be used to get a grenade into areas where it would be difficult to destroy (after next build) or see; and potentially allow offensive attackers to launch grenades from underneath the flag area.

Reproducability:
Always, but varies upon location, player skill, and patience.

To Reproduce:
Vertical wall surfaces are very easy to perform this on. Position yourself about 20-30 meters from the center of the corner, and at a 45 degree angle to both walls. Using regular grenades (i.e. don't hold fire yet) zoom, fire directly at the center. Adjust your fire left or right based on the bounce. When the grenade seems to immediately explode upon hitting the corner without falling, fire again and hold to arm. The grenade will get stuck and climb upwards.

Demonstration Video - 31M