(from Fundamentals of Biochemistry by Voet, 5th ed. — modified, replacing Pi by orthophosphate)
In this step of glycolysis, I'm not seeing where the $\ce{H+}$ ion on the product side is coming from. It seems to me that the G3P's aldehydic H is replaced by phosphate, and that H is given to $\ce{NAD+}$ to make NADH. So where is the extra $\ce{H+}$ coming from?