That's true! There are times that even you have set your limits but when something unexpected happen, you are being push to bet more or the allocation of your bets will exceed to what should be the amount of your bet, it will begin to ruin your plan and strategy, the good thing with experienced gambler though there are times that they experienced this kind of mistake they manage to re-assess the way they play the game and start to re-establish the original plan and if luck permits they will be able to recover and may win a little after some days of playing again.
I would call over-the-board budget spending in gambling a mistake, I believe it is a deliberate act by the gambler, take for example what we call a trading limit for example there are times that I will set a trading limit timer on my trading activities, but along the line, I always overspend and move beyond my set spending limit.
This overspending in gambling is not something new to most of us at the time it can also be a sign of the early stage of addiction both in gambling and trading since we tend not to be in control of the emotion that pushes us to bet more or increase our order by x amount above what we budget for such.