Brisbet requires you to submit the bet and then resubmit it, so when you are doing it quickly you may think that you submitted it but you have to see the screen reply of "bet successfully submitted " in the center of the screen.
I feel your pain. Similar type thing happened to me once or twice, once where I input a $1 key exacta over numerous hosses and the amount changed to $6 key. What happened was that the bet amount box was still highlited and when i moved my finger on the laptop pointing surface it changed the amount, and I didnot notice it when i placed the bet. Now, I try not to rush to get the bet in, if i get closed out, so be it.