There are 2 mystery wedges on Wheel. One is the $10,000, while the other is a bankrupt. Even P. Sajak will let the contestant know it's a 50/50 chance if the decide to pick it up. There are only a few bankrupt spaces on the wheel and are spaced apart. Some contestants seem so focused on looking at the puzzle, they seem to spin the wheel with no regard to where those are. Ones who call a letter that was already called are obviously not paying attention.
Clearly some puzzles are easier to solve then others. Every consonant you call may not be in that particular puzzle. If you know the puzzle and continue to spin in an effort accumulate more money, you may hit a bankrupt and lose out.