Truffle deployed() vs new()

Example.sol

pragma solidity ^0.4.23;

contract Example {
  uint256 public data;

  function set(uint256 _data) public {
    data = _data;
  }
}

Example.spec.js

const Example = artifacts.require('Example')

contract('Example', (accounts) => {
  let instance

  beforeEach('setup', async () => {
    instance = await Example.new()

    // second test will fail with `deployed()`
    //instance = await Example.deployed()
  })

  describe('test singleton', () => {
    it('should set data', async () => {
      assert.equal(await instance.data.call(), '0')
      const {receipt: {status}} = await instance.set(5)
      assert.equal(status, '0x01')
      assert.equal(await instance.data.call(), '5')
    })

    it('should not read previous data', async () => {
      // this will be `5` instead of `0` if using `deployed()`
      assert.equal(await instance.data.call(), '0')
    })
  })
})