Sadest problem with this one, was that they completely rewrote Spike's past hence why it was a bit of a let down for me.
For starters Spike said in two seperate occassions way before that Angel was his sire, in 'School Hard' he states exactly that and in the Angel episode 'In the Dark' he refutes that statement saying quote:
'I don't get my ring I'll stake me own sire right here and now!'.
That's area number one which is a bit of a letdown for me simply because they changed that around.
Area number two is of course when Spike says he was turned in the 1880's his math (and that of the writers) is very off namely because if he was sired in that time period and he's one hundred and twenty-six now well he would've been six years old when he was turned.
This of course ties into School Hard where Giles states that Spike is 'barely over 200' clearly if Spike wasn't that old then why are there records of him dating to before his turning?
Next is how Spike got the scar, unless the sword was made with holy water or blessed there's no possible way that wound would have not healed over.
Finally we have the whole quote in School Hard where Spike states that the last slayer he killed begged for her life either Spike was lieing about that or the writers have very poor memories.
The latter of course sounds alas about right, don't get me wrong the way Spike weaves a story was good as was his defiant attitude but is it too much to ask that they keep things true to context?
Not to mention Buffy's attitude towards him throughout season four and later on was pretty much intolerable.