What is the biggest mistake you've made in a relationship?
Pride -- no question about it. And that subcategory of pride, making the presumption that someone must be inferior to you simply because of surface details.
Case in point: When the White Guardian told me that I was to be traveling with the Doctor to locate the Key to Time, I did a bit of cursory looking into his record. Of course I knew who the Doctor was -- everyone did, at that point. A renegade who was once suspected of having murdered a Lord President, but who then submitted his name as a candidate for that very presidency in order to buy himself more time. One who then revealed the true murderer of the president, but was involved in some murky business in the Matrix which resulted in the death of that murderer...which inadvertently made the Doctor president since there were no other candidates in the running. And then he ducked out on his duties, and there had to be another election. Some claimed he was brilliant, others said he was a madman, others said he was an agent provocateur, and some said he was a fool in a rickety old Type 40 with a broken chameleon circuit, for Rassilon's sake.
Of course, the bit about the Type 40 TARDIS was true. And his Academy records were simply appalling, to boot. So I was only too ready to assume the worst, and I did. I'm very surprised that the Doctor didn't simply knock me on the head, nick the Locator wand from me, and leave me stranded on some primitive planet that hadn't discovered shampoo and conditioner yet -- I would have deserved that fate in spades.
By the time I realized that the Doctor was not an idiot, I was too proud to accept the fact that I had been horrendously wrong, so I kept acting like a frosty ice queen instead of relaxing a bit and making a proper apology. Over time I thawed out, but it wasn't until I underwent a regeneration that I really felt we had a "second start", and that time we got it right between us. But it was my own stupid pride that hindered everything at the start.