An ascending triangle can fail, most often through a breakdown below the rising support line or a false breakout above resistance on weak volume.
An ascending triangle may fail if the breakout lacks volume and conviction, leading to a false breakout, and the pattern can also break down below the ascending support line, which is a bearish signal that invalidates the pattern; this can happen due to unexpected negative news, a sudden shift in market sentiment, or simple failure of buyers to overcome sellers.
The failure rate is roughly 17% on daily equity charts, with intraday and crypto charts showing higher noise and false break frequency.
An ascending triangle that prints inside a broader bear trend has a lower hit rate, so use a trend filter such as price above a rising 50-period moving average.