ok what about this.

What happens if the tank turns and due to poor visibility and slams front first into a high rock wall. Everyone gets jerked around inside and the the gunner hits the fire button, causing a round to slam at point black range into the cliff wall?

If the gun barrel extends past the front of the hull I guess it would hit the rock wall before the rest of the tank and be damaged or destroyed before the round was fired. The round might explode in the breach, explode in the barrel or the gun may have been rendered incapable of firing. The first two events would obviously be highly detrimental to the tank's crew, especially the commander, gunner and probably loader.

This thread is starting to crack me up....thanx all....

Being an anti-tank buff, there are some good and bad sides to this coin. And some of this has already been covered,

Good
-When tanks are buttoned up, they can see about 1/10 they normally can
-tanks cant drive up greater 60 or 70 percent slope (look for steep hills)
-tanks cant go over trees over 18in thick at trunks
-ditches and cuts over 4 feet can get tanks stuck

Bad
-good tankers nearly ALWAYS have infantry support
-tanks move FAST! (well, comparatively)
-good tankers know the weak spots on their vehicles and protect them

...So going back to the original question with the single rifleman with a rifle and a rock; this would actually be a good idea for a scenario for the GM with writers block.
This isnt impossible, just very difficult. Imagine if you will, attempting to place the rock (maybe a chunk of scrap steel instead) somewhere in the tracks where it would throw the track. What kind of courage will a PC show to make this happen with the possibility of the tank making a track turn and turning the PC into a red chunky stain in the ground up dirt? Or climbing onto the back deck of a buttoned up tank; what if TC or Gunner traverse the turret 360? Could a PC climb onto the turret? Could someone hang on at speed and hard turns? If the tank decides to "scratch its back" using a building, can the PC jump off and hide again? Could the PC get a hatch open or just wait for armor crew to open it for him/her? Will the tank crew crap the bed and make a serious mistake?
All this of course assumes that the tank is alone and without infantry support, but what a game session that would be for an imaginative PC.
then of course theres always;
"Make a deal with em!"
"What kind of deal?"
"A deal deal!"
"....ok hustler...we'll try it your way"

And most tracked vehicles can't handle a side slope of more than 30 degrees -- try to trick it into going sideways across a steep hill and you can make it roll over. (I damned near rolled an M-113 once -- hell of a scary experience!)