Plumbing leaks: When pipes running behind walls burst or leak, they moisten the drywall. Eventually this moisture makes its way through the walls and produces wet spots.

If you notice persistent wetness or a musty smell, you may have a leak. Contact a professional plumber. If the problem is in its early stages, the plumber can replace only a section of the drywall.

Over time, the moisture may lead to black mold, requiring not only the services of a mold remediation company but also the removal of large wall sections.

This can be very costly. For example, 100 square feet of drywall can cost as much as $400 to replace, while mold remediation can range from $500 to $3,000.

— Angie's List