What are the possible errors with declarations?

There are two common possible errors with declarations array,

1. If you use a component without declaring it, Angular returns an error message.
2. If you try to declare the same class in more than one module then compiler emits an error.

Practical Code Example

// Common Declaration Errors:
// 1. Component declared in more than one NgModule
// 2. Declaring a service or interface in declarations
// 3. Forgetting to import the module exporting a required directive