When she mentioned she was a Gallifreyan, did anyone else think for a second she might have actually been The Doctor?

Nope. She was pretty obviously the Master, and by that point in the episode we'd already heard the Master's musical cue while the camera focused on her.

Going into the episode I thought she was the master, but when she said ''2 hearts, both of them yours'' to the doctor I misinterpreted it and thought they were literally his hearts - so then I did think for a moment she was him

No, never thought she was the Doctor. They did have me thinking she might be Romana for a moment or two but apart from that I always was expecting her to be the Master.

Throughout the episode I thought Master, Rani, Romana, then Valeyard - the 'there both yours' through me off.

Knew she was going to turn out to be a Time Lady. Wouldn't have been surprised if she turned out to be a sort of dark shadow of the Doctor - the unusual regeneration certainly gave room for that story. Equally not surprised though that she's the Master.

The abandoned her and hearts being his made me think she was Susan's mother.

For one horrible second I thought she may have been Susan.

I thought she was going to Rani.

I must have missed that - where was it, can you recall (I'll just watch it again anyway to try and spot it if you can't!)

I too thought misunderstood the comments and thought that Missy was a representation of The Doctor...

I thought Master, Master, Master, Master then the comment about abandonment came and I thought Romana as perhaps she saw being left in the E-Space as abandonment by the Doctor but then moments later I changed my mind back to the Master.

I thought she might be the Rani for a minute.

When she said Both of them yours I immediately though The valyard

With the line about being abandoned and both hearts belonging to the doctor i thought Romana for a split second, although looking back now I does kinda make sense for the master being sent back (abandoned) in the time war and both hearts being the doctor as a reference to their somewhat frenemy relationship (they have a very complicated relationship but i think in the sound of drums it was established it was a little bromancy)

That would have been dark.

Yes for a brief second I thought maybe she was the Doctor.

sadly it turns out she is the master, i was so depsearatly hoping for anythign else .

I was worried she was going to be the Rani so was quite relieved when she wasn't.

The fake-outs were quite cleverly done - Moffat deliberately played out every speculation there was going to be. "She's a robot! Oh, no she's not." "She's a scientist person, she must be the Rani! Hmm, not this time." "Two hearts, both of them his! She's Susan! Eww, grandfather snogs, no that's can be it, she's a future Doctor! Wait, no it's not that." "She's a Time Lady, abandoned by the Doctor - she must be Romana!" "Wait, she's WHAT?"

I did think it was Romana when she said it was Mistress.
Slight disappointment that so many called it weeks ago,though technically it could still be anyone I guess.

No. I just thought it was the Master having a laugh with the Doctor and that he will reappear at some time in a male body.

When we got into stuff about uploaded consciousness, I thought it was going to be a new version of River.

The Teselecta, perhaps?

I actually thought Jenny - the Doctors Daughter.

Two hearts from him and he did abandon her as he thought she was dead.

The thing is the Library was only the, ew Who version of the Matrix from Deadly Assasin which was a Master story so.

I've always assumed that the Library came from Tine Lord technology rather than the other way round.