Yeah, just that. Has Yoda ever turned to the dark side of the Force? Has he ever been tempted? Even just for a moment? Even just in his mind (I mean, clearly temptation is in his mind, but I mean tempted by himself, rather than from an external input)?
In the Clone Wars episode "Destiny", Yoda has to face certain trials prepared by the so called Force Priestesses to show he is ready/worthy to receive the secrets of immortality (keeping your consciousness when getting one with the Force/being Force ghost). The first trial is about leaving behind fear, here he encounters the manifestation of his inner dark side.
The dark side fights Yoda, but Yoda says that the shadow is not part of himself and rejects it. The more Yoda rejects his shadow, the more he seems to become it. Only when Yoda says that the shadow is indeed a part of himself, but that he would not let it go, does the shadow begin losing its power. Yoda then vanquishes his dark side.
Here is YouTube link to this scene.
Since the trial is about fear, we can assume that Yoda feared the dark side within himself, but could eventually overcome this.
Yoda plays with me not anymore. Yoda thinks me not worthy.
This line said by the dark side of Yoda implies that to an extent he did touch the dark side of the Force before.
Yet you spend your days in the decadence of war, and with that, I grow inside you. Know your true self, face me now. Or I will devour you.
This does not speak about the temptations Yoda experiences, but clearly states that he does experience temptations to turn to the dark side. Its main source is the ongoing clone war, which forces the Jedi into roles they shouldn't assume, see destruction and evil from the first row, even playing their part. According to the creature as long as Yoda takes part in the war, his dark side will grow. But in the scene Yoda realizes that accepting his dark side as part of himself, kind of accepting the danger of his fall instead of arrogantly neglecting it, is what makes him victorious in this never-ending fight.
Although this scene does not count as Yoda "turning to the dark side" or even on the contrary, I think it a valuable insight into Yoda's temptations and his relationship with darkness.
In the EU novel Yoda: Dark Rendezvous, we see Yoda trying to turn Dooku back to the light side of the Force. During this verbal interplay, Dooku also tries (futilely) to turn Yoda to the dark side of the Force. Clearly Yoda has given some thought to the subject over his 900 years and shows Dooku a glimpse of what he (Yoda) would be like if he devoted himself over to the teachings of the Sith. Suffice to say, it's not pretty.
At this moment Yoda turned, and Dooku gasped. Whether it was the play of the holomonitors, beaming their views of bleak space and distant battles, or some other trick of the light, Yoda’s face was deeply hidden in the shadows, mottled black and blue, so that for one terrible instant he looked exactly like Darth Sidious. Or rather, it was Yoda as he might have been, or could yet become: a Yoda gone rotten, a Yoda whose awesome powers had been utterly unleashed by his connection to the dark side. In a flash Dooku saw how foolish he had been, trying to urge the old Master to the dark side. If Yoda ever turned that way, Sidious himself would be annihilated. The universe had yet to comprehend the kind of evil that a Jedi Knight of nearly nine hundred years could wield. From the shadows, Yoda spoke.
“Disappointment like I not, apprentice,” he snarled, in a wicked, wicked voice.
Yoda is very clear about what he thinks of the dark side. Twice he tells Luke:
Once you start down the dark path, forever will it dominate your destiny.
This is not the sentiment of a Jedi who has had even a brief flirtation with the dark side. By the time of his death, Obi-Wan Kenobi had come to believe that going to the dark side was irreversible. It is not clear whether Yoda actually believes that too, but the assessment quoted above proves accurate for Darth Vader either way. Anakin Skywalker does return from the dark side, but there is no profitable life left for him to lead after his return, and Yoda recognizes this.