Engineers just love to put shields on circuits, mostly as a defensive measure against signals on the outside getting into and disturbing our circuits, but they also keep signals inside from getting out and this really makes the folks responsible for EMI compliance happy.
So what could go wrong? Well 10 years ago, not much. We were still working at frequencies up to 2.5 GHz for almost all the standard wireless applications. And shields generally only helped the situation. Today we consider “commonplace” to be below 6 GHz, and we may routinely need to lay out circuits that operate to 6 GHz even in consumer circuits (WiFi being most common application for those frequencies).
When we place a shield on a circuit board we are creating a conductive electrical cavity and this cavity likes certain frequencies or various TEM (Transverse ElectroMagnetic) modes. In other words it becomes resonant at certain frequencies. These resonance modes are usually bad.
It is possible to predict the frequencies where this starts to happen by using simple math. This article gives you this math and other tips.