I have the Unlimited Power node in the Static tree, making the automatic Zap from the Overload node cast a Lightning Blast instead. This appropriately consumes mana in addition to static charges, as explained in the tree (and I’m specialized in Lightning Blast, with the mana cost at 9 with a wand / 12 without a wand). I also have points in the Unstable node, in case that’s relevant.
However, when casting abilities puts me in negative mana and enemies are nearby, static continues to be consumed every second in large chunks (seemingly for the Overload/Unlimited Power ability) but no mana is consumed and no Zaps or Lightning Blasts are created.
I assume this is a bug. The most logical result to me would be no static being consumed at all while your mana is negative, and the ability simply isn’t triggered, just as if you didn’t have enough static.