Situational irony can be one of the more difficult types of irony to properly define, particularly when explaining it to another person. As such, it bears repeating that situational irony only occurs when reasonable or expected outcomes for a certain situation are either inverted or not met at all.
Say one day you were walking through the park and happened to chance upon Shaquille O'Neal out on a walk as well. If the two of you were to hit it off, and decided that it would be great to get some exercise together it would be something fun and friendly to do. Considering that Shaquille O'Neal is a basketball star, the two of you decide that you should both play something relaxing and new to you, like badminton.
If Shaq does a terrible job of hitting the shuttlecock, and loses the game it might be funny, it might even be hilarious, but it is definitely not situational irony (or any other type of irony). You may say, well Shaquille O'Neil is an athlete, is it not ironic if he fails to perform well when competing in a sport? The answer to that question is no, at least in this situation.
Shaquille O'Neil is definitely a professional athlete but he is not a professional badminton player. When you agreed to play badminton with him you did not expect him to be an incredible badminton player. Had your opponent been a professional badminton player, rather than Shaq, it would qualify as situational irony.
On the other hand, if the two of you agree to play a game of HORSE together and Shaq not only lost the match, but failed to even get 'H' that would be situational irony. As a professional basketball player, a person would expect Shaquille O'Neal to win the game, or at the very least do fairly well. By losing, and drastically so, Shaq would've done something that was the exact opposite of what you expected from him.