A mirror is read through self-reflection and self-awareness, sharing the same basic logic as a calm, clear lake elsewhere on this site — seeing yourself clearly is read as genuine clarity about your own current situation, not just a literal glance at your appearance.
A broken or cracked mirror is read as a fractured self-image, or a relationship or reputation that's taken real damage — one of the more visually direct symbols in this whole tradition, since the crack in the object maps so cleanly onto the crack in whatever it represents.
Seeing someone else's reflection where your own should be is read as feeling disconnected from your own identity right now, or being unusually influenced by another person's expectations or presence in your life. Multiple mirrors, or an unsettling hall-of-mirrors effect with many reflections at once, is read as feeling overwhelmed by too many roles or versions of yourself being asked of you simultaneously.
An old, dusty, or clouded mirror that's hard to see clearly through is read as a period of genuine confusion about yourself or your circumstances — not a crisis exactly, but a real lack of the clarity a clean mirror would otherwise offer.