Lucky Strikes which randomly occur during battle, will give you an extra 25% on the gold you steal. So you steal 80% by default, on a Lucky Strike you will steal 80% from the player, but then you get 20% of the players gold as a bonus, which pretty much means you will get the full amount of gold the player is holding. Here is a table that shows the chances of a Lucky Strike depending on your Sleeper Level.

When you lose a battle and a Lucky Strike occurs you will receieve 5 minutes worth of your opponents TBG, to help compensate for the attack turns lost. All bonuses are applied normally to the gold, at this given time though, lucky strikes on defended attacks arent logged.