If you are truly concerned about this happening behind your back (rather than making a comment about how easy it is to not recall adding something), you can contact me (neilblakeymilner at fb.com) and I will try connect you with someone who might be able to help you find out when you did it and whatever other information might be attached to that event (I don't really know what, if any, we keep on that event type).
I am not aware of any problem this has caused me for a long time, including on the sites you mentioned. Maybe there is some useful feature I'm not seeing at all because of the cookie restrictions I impose, but maybe they've just got better over time at not relying on cookies for things they shouldn't?
We are so afraid of Google and Facebook tracking our searches/web pages, yet we freely install plugins from 3rd party developers that can easily gather everything that Google and Facebook can get, and more. In theory, I could make a Facebook Disconnect 2, which secretly sends data back home about what pages have been visited, and nobody except the most vigilant (enough to read the source of the plugin) would know.
I have never, in 20 years of programming, ever made this mistake; nor have I ever had to fix a bug that was caused by it. I generally do insert the braces anyway on the basis of almost a cargo-cult theory that someone else might make this mistake, but only because of received wisdom floating around on newsgroups an the like.
Abstraction gets its power by using indirection: generalizations that stand in for specific cases. Making abstractions concrete involves a flattening of all these indirections. But there's a limit on our ability to automate the manipulation of symbols - we don't have Strong AI that is able to make the leaps of insight across abstraction levels necessary to do the dirty hackish work of low-level optimization
