It is disliked ( Makrouh) to say Wallah ( By Allah) or any word with the same meaning to prove a claim if you believe that the claim is genuine. The Hadeeth says: True oaths destroys the houses.
Obviously, false oath is a major sin and it is called the sinful immersing oath because it immerses the liar inside the hellfire.
If you thought that something happened and said Wallah, then discovered that you were not right, you did not commit a sinful act, but a disliked act. You need to inform every one who heard you that you have misunderstood.