Yeah, musty mildewy weed is ganky, lol. But also, if you leave insufficiently dried weed enclosed in a container too long, even paper bags, add a little too much heat and your bud starts to decompose, and even a little of this goes a long way to ruining the odor and flavor profiles, whether or not you actually develop mold/mildew.
Proper curing is one of the harder aspects to learn in this craft, just takes time/experience or the ability to follow instructions to a tee and extreme attention to details. I like to think of it like being able to create an excellent home brewed beer or wine, it can be learned over time, while some just have a knack for it.