In The Eye of the World, Thom attacks a fade, Rand and Matt both assume he dies. We learn later that he indeed lives through this, but the fight itself is a mystery. They see Thom jump at the fade and then a blue light, or blue fire explodes around him and the fade and they here him screaming. Later when Moiraine, Lan, and Nynaeve come to Whitebridge they see the town has buildings burned randomly through the city. As far as I know Fades don't use any form of the true-source, or any powers that would allow them to cast blue fire, or lightning. What caused this, was it Rand subconsciously using the one power, or was it something else?
There's no canon on this, but I've always assumed that one or more of the knives secreted about Thom's person were Power-wrought. You know that he's the kind of guy that would pick something special like that up when he had a chance. There are other scenes in which power-wrought blades, such as Lan's and Mat's, generate blue sparking when clashing with Thakan'dar-wrought blades.
Here are a couple of the cites I was thinking of:
From The Eye of the World:
[The Fade's] hand swept toward the hilt of the black sword hanging at its waist, but the gleeman's long legs covered the distance quickly. Thom crashed into the Myrddraal before the black blade was half drawn, and both went down in a thrashing heap. The few people still in the square fled.
"RUN!" The air in the square flashed an eye-searing blue, and Thom began to scream, but even in the middle of the scream he managed a word. "RUN!"
From The Fires of Heaven:
The Myrddraal's blade flickered like a serpent's tongue, darted like black lightning, but to counter Mat's attack. When raven-marked Power-wrought steel met Thakan'dar-made metal, blue light flashed around them, a crackle of sheet lightning.
When Thom Merrilin fought the Fade at Whitebridge, no one is sure about (i) where the blue lightning came from and (ii) how Thom survived the fight. Below is one explanation that no one else seems to have considered. I can't say it is the best explanation, but I think it is worth considering.
I have often speculated that Thom has a very weak ability to channel saidin, e.g., along the lines of how much Queen Morgase can channel saidar, but somewhat stronger than that. Let's not forget that Thom's nephew Owen had the ability to channel saidin, and that the ability does tend to manifest in the same bloodlines. Furthermore, Thom seemed even more upset over Owen's fate than is reasonable. It seemed as though Thom intended to help Owen. How could he do that unless Thom himself could channel?
A very weak ability to channel saidin might also help to explain Thom's uncanny accuracy and effectiveness with his knives and daggers.
Finally, in the very last book, "A Memory of Light", Thom also played a role that seemed beyond his abilities, when he was guarding a cave at the Last Battle.
The above explanation does not mean that other explanations are invalid. For example, even if my explanation is correct, it might also be true, at the same time, that Thom's blades were power-wrought, or that Rand unconsciously channeled.