People are often mistaken for cheaters by new players. Here's an example:
You got this dude sitting for hours, weeks, playing cs. His skills improve and he's pretty warm. He's played dd2 5000 times and knows for example the exact moment someone will be there or here based on what he sees on the radar, what sounds he hears, and based on what he'd do in a particular situation. He's developed a good perception, and you'll see him moving around, only to stop at some seemingly random place, firing a couple of shots through the wall, hitting a player. He knows he's there, or makes a pretty good guess, based on experience. He's been playing for hours every day for a long time and is pretty much unstoppable. Now along comes a new or casual player who plays dd2 for the fifth time in his career. He speccs one of these players and immediately decides this person must surely be using a third-party program, consequently calling him a hacker and a cheater. The experienced player thanks the newcomer for the compliment and carries on with his business.