Dragonfly bug?

gistfile1.js

// Coming in:
//
// weekday = 40;
// d = new Date(2012, 1, 1);
// utc = false;
// prefer = 1;
// callDateMethod with those arguments is equivalent to "d.getDay()" and will return 3;

// When the bug occurs, weekday will become 47, where in every other browser it should remain 40 as:
// callDateMethod(...) IS 3 AND 40 % 7 IS 5
// 3 - 5 IS -2
// -2 < 0 IS true 
// prefer === 1 IS true
// true === true SO
// 0 * 1 * 7 = 0
// 40 += 0 = 40

// console.log on each of these parenthetical statements will always produce the same result in all browsers,
// no discrepancy. It only triggers on the full line, (my fix was splitting up the statement, which it should
// have been anyway... the fact that all the "parts" are ok makes me guess it may be a parsing bug)
// There are 12 unit tests that run through this code before the final test triggers the bug.
// If ANY ONE of those test are removed it will not trigger.
// If Dragonfly is open it will not trigger. 
// If the same line is logged one line above it will not trigger:
// console.log(((callDateMethod(d, 'get', utc, 'Day') - (weekday % 7) < 0) === (prefer === 1) ? 0 : 1) * prefer * 7);

weekday += ((callDateMethod(d, 'get', utc, 'Day') - (weekday % 7) < 0) === (prefer === 1) ? 0 : 1) * prefer * 7;