I read about this some time ago and only remember a few details. I think Hannibal was one of the generals present. Basically two armies had been facing off for several days. Every morning each army would form up and essentially have a stand off. Then after staring at each other for some time they would simply break formation and go about their business. This went on for several days or weeks. Then one morning one of the armies formed up in the fog with a completely different variation in their structure. The opposing army formed up as they had been the whole time, unaware that the opposite army had altered their formations. They went to battle, and were then defeated because they didn't see the difference in their opponents battle formation which gave them a large tactical disadvantage.
Sorry if this makes little sense, but any insight to this would be much appreciated.
You're thinking of the Battle of Ilipa in the Second Punic War, in which Hasdrubal Gisco was the Carthaginian commander, while Scipio Africanus led the Romans. In the leadup to the battle, Scipio would array his heavy Roman troops in the center, with the more lightly armed Iberian allies on the wings. Hasdrubal matched this setup: heavy center, light wings. On the day of battle, Scipio switched it up, deploying his heavy troops to face Hasdrubal's light troops. Sun Tzu would describe it as hurling a stone against eggs. With the flanks sundered, the Carthaginians were routed and cut down.
It's possible you're referring to Marathon. The Athenians were arranged at Marathon for several days, performing sacrifices and sending messengers to call for aid, which the Plateaens responded to with a thousand hoplites.
The ten generals then held a vote on whether to attack, and the vote was split. The polemarch, after Miltiades - the strongest proponent of attacking - persuaded him, cast the deciding vote in favor.
...and when the polemarch's vote was added to the tally, the decision was made to join battle. And afterward, the generals in favor of the battle each in their turn ceded their day of command to Miltiades when the day came around for each to be in charge. But while Miltiades accepted this, he would not make the attack until it was his day to preside. Herodotus 6.110
During this period of delay, both the Persians and Greeks were maneuvering their auxillary and cavalry units in an effort to exploit a mistake by the opponent. I don't have the other sources in front of me right now, just Herodotus, and although he attributes the specific day battle was joined to favorable sacrifices and a quirk of Miltiades, other sources indicate something was going on with the cavalry units on the day of the battle. It's likely the Herodotus explanations are also accurate, in their way, but there was more going on with the preparations before the hoplites took the field and charged across a mile of empty ground in full armor, without the support of cavalry or archers. Herodotus 6.112