I suffered a lot from RJ in my former relationship and the experience has made me a lot wiser about it. I learned that it can much more than pestering involuntary jealousy, but rather your instincts trying to warn you about things your conscious self doesn't pick up...

When I and my ex met we immediately became good friends. We would hang out and text lot, which turned into flirting and dating, and eventually she invited me to spend the night at her place. Our chemistry had markedly intensified and it seemed to be the ultimate que and an opportune moment to propose a relationship. The date rolled around and we had an incredibly good time, but when I confessed my feelings to her - fully convinced she'd respond in kind - she hit me with a mother-of-all-bombshells - she still wasn't over her ex... (then why date me for months and invite me over??? What was I supposed to think?? And, no, she didn't want to fuck). Though upset, I told her that she was worth waiting for if she wanted a relationship with me, but that until then I'd want us to spend less time together. She was in turn upset that I wanted to chill our relations, but agreed. I spent the night... we were sleeping next to each other.... I disappointedly left the next day.

A few days later though, she convinced me to continue seeing her, despite being ambiguous about her feelings for me. I really liked her and well...promptly gave in. We continued dating, having lots of fun together, and sleeping over at her place but much to my dismay she was never ready to become an item. Furthermore, I caught her texting her ex once while she was with me, and I also discovered him in some of her social media posts - these things brought me on the verge of giving up but every time she'd persuade me that it meant nothing and that I shouldn't get worked up. Once again, my feelings for her, the good times we were having, and my overall gullibility at the time convinced me to stay. This went on for months, until at last she was ready. Over the next two years we had many adventures, became an inseparable team and each other's best friends.

However, even though I was in my dream relationship and practically drunk on the euphoria, I started having recurring, haunting thoughts about her and her ex... I had nightmares about them and it'd frequently occupy my mindspace making me unable to focus on anything else. We all know what RJ is like, and I was experiencing it in full swing. I did my best to shield my girlfriend from it though and suppressed it as much as possible. I believed wholly that it was my problem and that I shouldn't burden her with it.
Worst of all, I couldn't understand it...my gf was with me not her ex, she'd cut him off...I was her partner and best friend, she was just as euphoric as I was...why should I be jealous?

This constant torment went on for the entire duration of our relationship, though declined in the latter year somewhat. All the while I kept to myself... I didn't want to diminish myself in her eyes nor display overt insecurity, and resisted incessantly asking about her ex as is typical of those with RJ... Though it was always there, I learned to live with it. I loved that girl too much to spoil our relationship because of some douche in her past.

Now, how is it that despite being such a solid couple for so long we aren't together anymore? Two years on, she cheated on me and promptly replaced me with another guy... I never believed she had it in herself to cheat but lo and behold... I was shocked, devastated, and in disbelief for a long time, until eventually I found out that back while we were dating...she was still together with her ex. She was cheating on him just as she had cheated on me. Every time she convinced me not to get worked out about him (them texting, their pictures etc)...she was lying. She was never as committed to me as I was, only acted like it to keep me around. My RJ had the last laugh...

I learned that even though I trusted her on the surface, subconsciously I didn't, but I suppressed my gut feelings into oblivion. Back then I didn't have any irrefutable evidence that she was cheating, I wasn't in a position to accuse her of anything, and was too in love to leave her without a clear reason why.

My verdict in a more narrow sense is that any talk about an ex early on is a glaring red flag, and in a broader sense - we have these instincts for a reason. Of course, they aren't always right - sometimes the subconscious can overcorrect and make you paranoid - but in their presence one should be more vigilant and reserve healthy suspicion. If I had broken up with my girlfriend over RJ I would've missed out on a lot of the fun and would've asked myself what it could've been for the rest of my life. Suppressing it is not correct either, as it allows one to become too attached to their partner and, in my case, ignore signs of disloyalty. I now know that if I experience RJ again, I ought to chill my euphoria and live more in the moment, as opposed to fantasizing my entire future with that person.

RJ can be a legitimate warning about your partner, but could also be a sign that you have placed them on too high a pedestal.