Callbacks & Callback Hell

When callbacks are executed in reverse order
When program flow is controlled by the framework/library rather than your code
Inversion of Control (IoC) refers to the transfer of control from your code to external code that calls your callback. Implications: 1) You trust the external code to call your callback appropriately, 2) You have less control over when and how your code executes, 3) Potential issues with callback execution timing or frequency, 4) Security considerations when callbacks have access to your scope, 5) This is one reason why Promises were developed, as they return control back to your code.
When callbacks are used for error handling
When callbacks are used synchronously