Example you know that a particular team win in the match and you still play against the team. Like PSG vs Chelsea. That was an avoidable loss for the side of the gambler. And many gamblers loss the game and where in sorrow and painful.
One can have a feeling, but it will never be certain unless the result unfolds. In this case, you may think that one team may win, but it is not yet absolute until the team actually wins. It is all but an assumption, so betting against your own assumption may not be wrong or a mistake if the result is the one you have chosen.
And unavoidable mistakes are those games you bet on that you don't know which team would win and you play base on the previous performance of the two teams.
As I stated above, unless the result is final, any bet you made will be an unavoidable mistake if the result says otherwise. No one can predict the future, so I think your example has some flaws in it.
Most cost of mistakes in sportsbetting is probably financial pressures. Like one example in the earlier reply, he failed to separate family expenses and used it to bet, thinking that he would win that time, but sadly, he did not. So by that time, he have to look for another means to replace the money that was spent.
Another mistake is failing to cash out in time when betting in a parlay, waiting for the last part to unfold when he has clearly won a significant amount. Greed probably plays a big part on the stated situation that cost him a good amount of money.