Starcraft has some strange glitches and hacks that some players exploit mercilessly. Here's an overview:

"The flying SCV glitch"
First seen in versions 1.06 - 1.09, this allows an SCV to fly over buildings and water if the scv is stuck and is commanded to build a building. In addition, the building can be built anywhere; I have once built a missle turret ON TOP of the opponent's observatory! This, I believe, has been fixed in version 1.10.

"The drone crash glitch"
The drone glitch, if applied correctly, will cause any player who sees the drone to crash if that person manages to see the drone. I don't think I should actually post this in public, but I will tell you how to do this glitch anyway. All you have to do is to burrow your drone the instant it transforms into a building. This requires precise timing, and you have to do it without watching your drone (so use your minimap) if you want to hurt only your opponent. I've myself have immediately terminated the program by seeing my own drone burrow.

"The dancing unit glitch"
If unit is stuck, but is provided enough space and is commanded to move somewhere, it will begin "dancing" in that area. This more often happens with units that are larger than they appear. An example of this is the Protoss Dragoon, which is a "large" size unit, but looks relatively smaller than other large units such as the Siege Tank. Sometimes you may want your dragoon to fit through an open space between two gateways, but instead it just dances around, causing you to lose time. The best way to avoid this is to always leave enough room for all your units, or make sure you always have a transporter (dropship, shuttle, overlord) in case a unit gets stuck.

"The Siege Tank glitch"
There is actually two possibilities: 1)  You can destroy your siege tank if you liftoff a building, then have a tank in tank mode, change it into siege mode, and RIGHT when its changing, you land your building on top of it, and your siege tank will be destroyed. 2) The other possibility is that the siege tank ends up being ON TOP of the building! Look below for a screenshot.

"The Username Color Hack"
This allows you to use different colors for your username. Check out below, an interesting username "closed" in rainbow colors!

"The Stack Hack"
Another relatively popular hack has been applied recently- the StackHack, which turns off the lights in Starcraft allowing you to stack buildings together. This was first used in a Use Map Settings game in variations of Turret and Photon Cannon Defense, but has now became a hack. Of course, I'm not telling you to try out these glitches and hacks, but its just to inform you. 

"The Map Hack"
All games have this...... so just be aware. Many games will lag in response to a map hack, so if your game is lagging, it is a possiblity. However it isn't all the time. The best way to counter a map hack is to not worry and stick with your plan; don't attempt to pull any strange tricks because your opponent will be able to see you. However, pulling fast tricks is great because you can send your forces into battle, and have your opponent focus his attention to the battle, while you do a trick.