Yī gè xiǎoxiǎo de shūhu niàngchéng le yánzhòng de shìgù.
A small oversight led to a serious accident.
Tips
memory
酿 originally means to brew (wine or vinegar), a slow fermentation process. 酿成 uses this metaphor: a problem 'ferments' over time into something serious. Always used for negative outcomes: 酿成灾难 (brew into disaster).