In the alpha editor, a for loop is generating the error "Uncaught ReferenceError: loopProtect is not defined". We confirmed this behavior on two machines, both OS X. I was able to test multiple examples, and examples with multiple loops were working (see, the "Iteration" on the p5.js site), but when that example was modified to have a single loop, the error returned. Here's an example to test:

function setup() {
  createCanvas(720, 360);
}

function draw() {
  for (var i = 0; i < 10; i++) {
    rect(50, i * 10, 475, 10);
  }
}