I think most fast food joints "special" sauces are mostly the cause of havin the shits.
trust me you're right about that, I usually leave the sauce off... I worked at Subway for a couple years as a teenager, they didn't give a FUCK about their sauce bottles, and those were right in front of the customer whereas BK and other places are hidden from view... muthafuckin science projects & shit growin at the bottom of the bottle.. shits were never cleaned, just refilled as they were gettin low so you'd have bottles with 2 - 3 year old mayo remnants, not to mention the amount of times that the bottles were left out on a back counter waiting to get filled in the middle of a rush, gettin all warm and funky...
STAY AWAY FROM THE SAUCE, or get packets if they're available