Gold Bug - Click on a peasant, right click on a mine, then build a town hall before the peasant has a chance to get to the mine. You will receive 100 in gold.
Lumber Bug - Have a peasant take a whack or three at some trees before building a town hall or a lumber mill. You will receive 100 or 125 in lumber (depending on what you build).
Tree/Wall Bug - If you have explored an area, you can keep track of your opponent's progress. If a tree is cut down or a wall is built, it shows through the fog of war.
Skeletal Bug - Have a death knight raise the dead, then put the skeleton into a transport. Eventually, the skeleton will run out of hit points, and the next unit you train will show up inside the transport.
Unexplored Area Bug - Take a peasant and right click on an unexplored area. Then look at the buttons to the left. If "Move" is highlighted, then the area you clicked on is not trees. If "Harvest" is highlighted, then the area you clicked on is either trees or a gold mine