參照：

這個梗,好笑!

不知耶！農曆七月還沒到啊！

謝謝版主回覆！
Help 中的項目都已檢查過，沒有問題，
我後來試了三次
Select device--> PIC18F4431
Connect debugger
Select device --> PIC18F4550
Connect debugger
似乎重新download ICD2 的 firmware 解決了我的問題，之後都未再發生，不過真的搞不懂為什麼會這樣。

ICD2 help 目錄下的說明，或到右上角搜尋"ICD0083" 裡面有很多類似討論。

General Debug Failure Actions
The MPLAB ICD 2 was unable to perform a debugging operation. There are numerous reasons why this might occur.

Top Reasons Why You Can't Debug
1. The oscillator is not working. Check your configuration bits setting for the oscillator.
2. The target board is not powered. Check the power cable connection.
3. Verify whether or not the device requires a header. From the MPLAB IDE main menu, select Configure>Select Device, select the device and check under the Debuggers, ICD/ICD Headers to see if a header is required.
4. The MPLAB ICD 2 has somehow become physically disconnected from the PC. Check the USB or COM communication cable connection.
5. The MPLAB ICD 2 has somehow become physically disconnected from the target board. Check the communications cable connection.
6. The device is code-protected. Check your configuration bits setting for code protection.
7. You are trying to rebuild the while in Release mode. Select Debug in the Build Configuration drop-down list on the Project menu, then rebuild the project.
8. MPLAB ICD 2 is selected as a programmer, and not as a debugger, in MPLAB IDE.
9. MPLAB ICD 2 to PC communications has somehow been interrupted. Reconnect to the ICD in MPLAB IDE.
10. The target application has somehow become corrupted or contains errors. For example, the regular linker script was used in the project instead of the MPLAB ICD 2 version of the linker script (e.g., 18F8722.lkr was used instead of 18F8722i.lkr). Try rebuilding and reprogramming the target application. Then initiate a power-on-reset of the target.
11. Other configuration settings are interfering with debugging. Any configuration setting that would prevent the target from executing code will also prevent the ICD from putting the code into debug mode.
12. MPLAB ICD 2 cannot always perform the action requested. For example, the ICD cannot set a breakpoint if the target application is currently running.
13. Brown-out Detect voltage is greater than the operating voltage Vdd. This means the device is in Reset and cannot be debugged.
Other Things to Consider
1. It is possible the error was a one-time glitch. Try the operation again.
2. There may be a problem programming in general. As a test, switch to programmer mode and program the target with the simplest application possible (e.g., a program to blink an LED.) If the program will not run, then you know that something is wrong with the target setup.
3. It is possible that the target device has been damaged in some way (e.g., over current.) Development environments are notoriously hostile to components. Consider trying another target device.
4. Microchip Technology Inc. offers myriad demonstration boards to support most of its microcontrollers. Consider using one of these applications, which are known to work, to verify correct MPLAB ICD 2 functionality.
5. Review the operational theory of ICD operation to ensure proper application setup. (MPLAB ICD 2 Overview)
6. If the problem persists contact Microchip.

以ICD2進入Debug mode 時，會一直出現
ICD0083: Debug: Unable to enter debug mode. Please double click this message for more information.
同一個IC座插入PIC18F4431則可以使用Debugger，且動作正常，請問是什麼原因呢？
(若只是單純Programming，兩個IC都可正常運作與執行。)