Exactly what you'd suspect.
This did happen in the episode you remember: "A Hundred Days", from SG-1's third season. Jack is trapped on an alien planet after a meteor hits the planet's Stargate. SG-1 has a hard time establishing contact, because it turns out that the gate has fallen horizontally; they send a MALP through, but it just falls back into the event horizon as soon as it arrives, and is destroyed.
Davis: MALP should be arriving at the Edora Gate in three...two...one. Receiving telemetry.
[The screen goes blank]
Davis: No wait, wait. We've lost it. There's no signal.
Hammond: What's happening?
Davis: Transmission interrupted at the source.
Carter: Play back the visual. Woah, there. See?
[the screen shows a cave has been burrowed out of the rock by the vortex]
Teal'c: The Gate is horizontal as you thought, Major Carter.
Carter: And the MALP just slipped back through the event horizon. It means the vortex would have dug partway to the surface.
Stargate SG-1 Season 3 Episode 17: "A Hundred Days"
The reason Teal'c needs to go through with a piton is exactly that: he needs to keep himself suspended above he event horizon, or else he'll be vaporized.
Would this be a good defense?
It would be better than nothing, but not nearly as good as an iris or coverstone. If you buried your gate in a horizontal position, you're still vulnerable to someone doing what Teal'c did: hanging inside the cavern created by the vortex, and digging their way out. Granted Teal'c probably wouldn't have made it out if Jack hadn't met him half-way, but it's a demonstrable flaw.
If you had it sitting out unburied, then there are at least two ways someone could successfully use it to get to your world:
Self-propelled machines. Stargate: Atlantis introduced us to the Puddle Jumpers and Wraith Darts, both of which are small enough to fit through an active Stargate. You also wouldn't be able to prevent UAVs (which SG-1 uses on occasion) or rockets/missiles from getting through
Jumping. This seems like a facetious comment, but we've seen that the Stargate preserves your momentum. In theory, someone could jump into their Stargate, and their arc would carry them far enough over the edge of yours that they'd arrive unscathed. It would be dangerous, but theoretically possible.