I don't know if the metaphysics check out, OP. I mean, is knowledge predicated on observation? Now, let's say your best tree and ex tree fell in the woods together, would they make a sound without you bumping into them?

OP, trust your gut. If you sense that your best friend was not there "for the right reasons" (I watch The Bachelor/ette way too much) then you should consider having a serious conversation with your friend about her intentions. Speaking from personal experience, I found out that my friend was regularly having lunch with my ex, and at first I brushed it off as nothing (in denial of the gut feeling that something was wrong). Eventually, it all blew up when this friend and I were going out one night and she decided to bring my ex *without ever asking me if I was okay with it*. I have not spoken to her since that night. I hope that you never have to experience this same kind of betrayal. Trust your gut. Talk to your friend before it's too late.

Hold up. Is your birthday coming up soon? This just happened to me from the opposite perspective

What's the issue? Just because you guys broke up means they can no longer be friends? You sound like one of those people where you expect all your friends to hate your ex, just because they're your ex.

I hang out with my best friend's ex a bit. She's cool.

So... your friend is also friends with your ex? So what?

I think we need a few more details. If you were all friends then before the break up then it was ok for him/her to see your ex. If you all weren't friend then whomever was friends friend should have checked with the third person

So for everyone attacking OPs FML. Let's go ahead and make up a scenario for everyone to see how OP should have at least got the heads up that the ex and BEST friend (which does make a difference) were seeing each other on a friend and/or non friend level (meaning a sexual level). Okay so you were dating this person for x amount of time and you were head over heels for them and something happened whether minor or not that caused you two to break up. X amount of time passed by and you come to find out that your BEST friend, keep in mind he/she knew about that relationship and knew about your feelings, was meeting with your ex in secret. I say secret because you had no clue about it and if that truly was your best friend, then you should have known about it because a best friend is someone you can share everything with. What you're telling me is, you wouldn't feel any type of distrust, hurt, betrayal, etc.? You wouldn't feel any type of way? Well it doesn't matter anyways what you feel cause clearly OP isn't you and he/she felt some type of way. So be a little bit more considerate of OPs feelings cause clearly their friend didn't.

Why is this an FML, except maybe for your friend, "Today, I realized my best friend thinks she needs a say in who I go to lunch with. FML" I don't even know what to vote on this, because I don't really see why it should matter

I can't believe how many people don't see this as a big deal. No man who has ever been with my best friend could ask me out to lunch nor would I entertain the idea of asking them. My best friend is like a sister to me and I can't imagine being remotely interested in any man shed dated, just too gross / awkward.