TL;DR: Top 5 rejection reasons globally: (1) Insufficient funds (2) Weak home ties (3) Incomplete documents (4) Failed interview (5) Previous overstay.
Country-specific: UK (TB test, NHS surcharge), USA (214b intent), Canada (travel history), Schengen (insurance, itinerary).
Prevention: Consult expert before applying, gather complete documents, practice interview, show strong home ties.