Is Yoda a Sith Lord because he spoke of an 'absolute' with Luke in The Empire Strikes Back?
For this writer, one of the best moments in Revenge of the Sith is the duel between Obi Wan Kenobi and Darth Vader on Mustapha.
They have some back and forth to which Obi Wan observes, "Only a Sith lord deals in absolutes" as a response to Vader's suggestion that Obi Wan is either with him, or against him.
Kind of like what George W. Bush said in his response to 911 at the time. That's the groove George Lucas was going for anyway.
This line is designed to show that Anakin has indeed turned to the Dark Side as he is in fact dealing in absolutes as would a Sith Lord. Anakin had chosen to side with Palpatine to try to save Padme's life.
He is lost.
That was the only choice he felt he really had and anyone who was going to get in the way of that happening was clearly against him.
The quote has been known to cause some confusion as some people interpret the actual line as an absolute in itself, suggesting Obi Wan was a Sith too!
And yes the quote is an absolute - but there is a subtle yet crucial distinction - Obi Wan is not 'dealing' in an absolute, instead he is making a fairly keen observation about Vader's behaviour at that moment in time.
He is not contradicting himself, in fact despite what some pundits have suggested, he's speaking very good English.

And now we get to the bloody internet meme that's running around that keeps suggesting Yoda's now immortal line from Empire Strikes Back "Do, or do not, there is no try" is an absolute statement and thus Yoda is a Sith himself!
Um, no.
Yoda is not dealing in absolutes, he is making a statement of fact to Luke.
Just like Obi -Wan did with Anakin about his attitude.
What he's setting is a challenge to Luke - if your mind thinks you are trying, you are not actually achieving.
If your mind thinks you are actually doing the thing you want to do - there's a greater chance of success. And that is what Yoda was trying to convey to Luke.
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Yes, I know this idea about Yoda a joke and those who share it are really just trying to have some fun, and maybe poke some fun at Lucas and Johnathan Hale's writing abilities.
It's kinda like Storm Troopers missing all their shots...
I wrote this correction of sorts for those who might be missing out on the joke.
Cool to do the former, wrong to use this example to prove the later.