"Cannot halt target after reset" error message

Why am I getting a "cannot halt target after reset" error message?

This error message is displayed when the debugger is unable to make the ARM stop running and enter debug mode, typically after carrying out a reset. This could be caused by any of the following:

• Unreliable JTAG Connection — Try reducing the JTAG clock frequency or reducing the target interface's cable lengths as much as possible.
• Incorrect support package — Make sure you are using a CPU or board support package intended for your part.
• Incorrectly connected reset signals — Typically the nSRST signal should be connected to the target's core reset signal, it should not be connected to the JTAG TAP reset (TRST) signal.
• Invalid code has been executed — This may be caused by your program not being loaded correctly, a problem with the target's memory or a bug in your program. You can check the download and memory by verifying the download.