As part of http://trac.webkit.org/changeset/121351, I landed LayoutTests/css3/calc/transitions.html that uses the transitions test framework.

It is failing flakily, with the error "CONSOLE MESSAGE: line 263: TypeError: 'undefined' is not a function (evaluating 'callback()')" See http://test-results.appspot.com/dashboards/flakiness_dashboard.html#showExpectations=true&tests=css3%2Fcalc%2F for results on other platforms - it fails on Mac, Win and Linux.

Line 263 is in this function in http://trac.webkit.org/browser/trunk/LayoutTests/transitions/resources/transition-test-helpers.js:

function waitForAnimationStart(callback, delay)
{
    var delayTimeout = delay ? 1000 * delay + 10 : 0;
    // Why the two setTimeouts? Well, for hardware animations we need to ensure that the hardware animation
    // has started before we try to pause it, and timers fire before animations get committed in the runloop.
    window.setTimeout(function() {
        window.setTimeout(function() {
            callback();  // <-- line 263
        }, 0);
    }, delayTimeout);
}

which is called from one place:

function startTest(expected, usePauseAPI, callback)
{
    if (callback)
        callback();

    waitForAnimationStart(function() {
        runTest(expected, usePauseAPI);
    });
}

and the callback is always provided.

More data from my local testing on MacOS 10.6.8:
 - using --run-singly with NRWT I can't make transitions.html flake
 - I've tried dumb things like renaming callback to callbackfoo - the error still flakily occurs, complaining about callbackfoo