I was just making myself a sandwich, and got out a jar of commercially-made mayo that admittedly has been opened for a while (refrigerated, of course!). It looked, smelled, and tasted OK, but was runnier than I remember it being. What happened, and did I just make myself a toxic sandwich? 
A bit of a guess (though I used to work on mayo development projects): mayo is an emulsion. Over time, the emulsion starts to break and the mayo becomes runnier. If it is not obviously spoiled, it should be OK to eat.
Damn, I noticed this about a year ago I think. I always buy the same kind of mayo (Best Foods of course), and I noticed that it separated kind of - there was oil-looking stuff and mayo looking stuff that seemed to be thinner than usual. The first time I just attributed it to the jar being a bit old (it wasn’t expired though).
Since then every jar has been like that - the mayonnaise is thinner, and separates after a month or two. My theory is that they’re no longer using trans-fat in the mix and whatever they are using now has a different consistency and different properties.
I can assure you that they are not adding trans fats to the mayo. There is a very strict standard of identity for mayo in the US. For real mayo, they are using soy bean oil, eggs, lemon (or lime) juice, vinegar, salt, sugar, and some spices (usually mustard). Oh, and EDTA to prevent oxidization of the oil. I think that is about all that they are allowed by the standard of identity.
My guess is that the emulsion is just coming apart. That is when you see oil separation.
Hmmm.
I did toss the jar in question. There wasn’t that much left in it, and while it tasted OK, I didn’t like how it sogged into the bread.
It may have become frozen at some point. It may also have become over hydrated. This can break down the emulsion.