A Cisco Catalyst 3560 switch that runs Cisco IOS Software Release 12.2(20)SE4 or later reloads or resets with the Debug Exception (Could be NULL pointer dereference) Exception (0x2000)! error message.
This problem occurs when the Powered Device (PD) is detected or classified as an overcurrent class.
This issue is documented in Cisco bug ID CSCsa72400.
As a workaround, do not connect 802.3af non-standard class PDs, or even bad or loopback cables, to the switch because the switch can detect the class incorrectly.
In order to resolve this issue, upgrade to Cisco IOS Software Release 12.2(25)SEA on the switch. Alternatively, upgrade to the latest maintenance release, which can be downloaded from Cisco Downloads.
Refer to the Debug Exception (Could be NULL pointer dereference) section of Catalyst 3750 Series Switches Troubleshoot Common Issues for more information.