What common mistake do developers make with Relationships, why does the mistake cause problems, and what is the safer approach?

What common mistake do developers make with Relationships, why does the mistake cause problems, and what is the safer approach? One recurring problem with relationships is applying it without understanding the boundary it protects. Treating an Eloquent model as if it were just a database row while ignoring relationship, casting, validation and authorization boundaries. In addition, Models can define relationships such as hasMany, belongsTo, hasOne and many-to-many associations. Safer engineering approach: keep the responsibility in the correct layer, use the framework's supported API, validate and authorize inputs, avoid hidden side effects, and verify the behavior with tests.