When I added the liquidity of YFI and usdc, I approved YFI ,then I was not asked to approve USDC immediately, but skipped this step and let me directly add liquidity. I think this is very insecure.you can see thx https://rinkeby.etherscan.io/tx/0x8ba72406c2d0443e4e6d94bb5c1ba0bf816d466d87d3d24bf966c2c192b9eb12 (approve token) and https://rinkeby.etherscan.io/tx/0x0f2ab2528d4aea1a655e891d98e6a05679e7b571b504e4adfe15d7ec3908e09b (add liquidity)