Seems like a natural target for a strike with all of that metal. Especially if mounted. If there are, what was the effect upon the scenario he was in (e.g., battle)?
EDIT: Thanks for the Faraday cage answers, especially ArcAttack playing Iron Man, very cool. I understand the concept and my question wasn't necessarily asking whether it killed the armor-bearer, but more has it happened and what effect it had on the situation he was in. Thanks also for the clarification that metal doesn't attract lightning. However the fact that a knight was usually mounted could potentially make him the highest object in the field. Especially if he's carrying a lance or a banner.
Metal does not actually attract lightning, that is a myth. The amount of metal in a suit of armor may divert a lightning strike to follow the quicker path to the ground, but only if the bolt would have struck immediately next to the person inside already - in which case the person would have effectively been hit by lightning anyway.
For what it's worth: wearing a head-to-toe suit of metal will protect you very well against a lightning strike. Giving the electricity a low-resistance path to ground will result in very little of the current taking the high-resistance path through your body.
So, a relevant question for medievalists: Which forms of armor would have a continuous metal path from head to toe? I'm sure a full harness of advanced plate armor would work, but would earlier forms of armor like maile be linked together over the whole body?
The Roman Emperor Carus may have been struck and killed by lightning while campaigning against the Persians. The Historia Augusta notes that there was a thunderstorm when he died, which may have resulted in a lightning strike, but also states that Carus may have died of disease instead.
There are some very interesting responses in this thread, but I would ask new commenters to consider whether they have information that directly addresses the OP's question, which is:
Are there any recorded instances of a knight in armor being struck by lightning?
And not interesting but unrelated questions such as whether armor acts as a Faraday cage, etc. So far only /u/LupusLycas has addressed the original question.
Thanks!