Back in the days when Cowboys played poker in saloons, you would always get some cowboy willing to try his hand at the 'String Raise' or 'String bet'. Basically a string raise goes down with the cowboy saying something like: "Mighty big bet their mister. I'll just see your 100," then the cowboy briefly pauses, looks at his opponent then grabs another stack of chips and says "...and raise you 200!" If you try that in a casino poker room at least 1 of the players around the table will call 'string bet'. Calling a bet then attempting a raise, is illegal in poker because it allows you to gauge your opponents reaction before you commit a lot of chips.

Only thing worse than being accused of this is when your opponent says "That's OK, the raise can stand," then you know you're in big trouble.