For example, I don't know if they've fixed this, but you used to be able to beat the AIs 1v4 on the highest difficulty. There was a bug where the AIs would surrender individually even though they were on a team. So with an all-out rush you could get two of them to surrender almost immediately by running 2-3 zealots through their mineral lines. Getting the third one to surrender required a bit more fighting, and then the fourth was more of a normal game.
The AIs also used to keep their entire army in one big death ball, and if there were 3-4 fighting units in their base they would send the army home to defend. Even if their army was 10x bigger than yours and they'd win the base race. So you could hop reapers into and out of their bases to prevent them from attacking your base indefinitely (because they kept running home). Also you were doing pretty serious damage with the reapers while they ran back.
Another exploit was to start building a cheap building in their base area at the start, somewhat far away from the mining line. They would send a ridiculous number of workers to attack your building. So they'd lose significantly more money to lack-of-mining than you did for building and cancelling a pylon or whatever.