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Sorry for asking/disturbing/bothering/etc (you) again. Actually I wanted to ask about the yesterday lecture.

The thought process in the second sentence is taking place in present context - I still want to ask, I still have that question(s) in mind, therefore one may argue using 'want' in place of 'wanted' is a better choice. Using 'wanted' is perfectly okay here and this is how it mostly appears. In such contexts the say in the second sentence is indirectly building on the thought expressed in the first. Consider this loose paraphrase, 'Actually I wanted to ask (and that's the reason why I have disturbed/bothered you again'). I wanted to ask and that's why I decided you to bother you again, I'm completely aware that it wasn't really a good idea. The use of 'wanted' perhaps suggests politeness. What is your explanation for this?
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Jackson6612Sorry for asking/disturbing/bothering/etc (you) again. Actually I wanted to ask about the yesterday lecture.
When I say this or something like it, I mean:

When I interrupted you just now, it was because I wanted to ask you a short question about yesterday's lecture. (the lecture yesterday)

It is politely making an excuse for the interruption.
The present tense (want) would be considered rude, because it implies this:

Now that I've rudely interrupted you and have your attention, I'll use the opportunity to ask you a question.
Comments  
Thanks a lot for the explanation, and the correction.