The mothership's barrier throwing items left

Verified against the spelunky.ai port v0.1.11 on 28 October 2026.

The mothership's barrier keeps the player away from the Alien Boss, throwing him back the way he came. It throws items too, but always to the left, the side the player comes from, whichever side the item reaches it from. So an item from the boss's side, behind the barrier, is thrown on through it.

Behind the Alien Boss, a bomb at the foot of a shaft the player has dug breaks the rock holding the free Jetpack and blows it left. It hits the Alien Boss on its way, costing it a hit point, and reaches the barrier, which throws it on to the left, and it lands beyond the barrier, which still stands ^[1].

The barrier's collision with an item lights a Bomb, and then throws the item, whatever it is:

other.xVel = -rand(4,6);
other.yVel = -2;

xVel is always negative, to the left. Its collision with the player tests which side he's on, and throws him away from it:

    if (other.x < x)
        other.xVel = -6
    else
        other.xVel = 6

The item's has no such test, so an item that reaches the barrier from the right, the boss's side, is thrown on in the way it was going ^[2].

Get an item behind the barrier and send it left into it, with the barrier still standing. That's the hard part: the barrier comes down when the Alien Boss dies ^[3], and when its emitter is destroyed: by an explosion, a bullet, or an item faster than 2 pixels a step ^{[4] [5] [6]}, or once it's on screen with the block above it gone ^[7]. So bombs have to go off well clear of both. The replay, with seed 63, reaches the mothership on level 10. Four bombs dig a shaft down the craft's right edge, and the last breaks the rock holding the Jetpack and blows it left: it hits the boss, meets the barrier, which throws it on to the left, and lands beyond it. The barrier, its emitter and the boss stand throughout ^[8].

The spelunky.ai port reproduces it exactly: it's in docs/quirks.md's list of the 1.1 source's bugs ^[2], and the oracle's fingerprint of the replay pins the Jetpack's path. No option turns it off.