What has gone Rong with my UNO?

Since 17 January 2017, I have been playing with Arduino Forum with my two co-players: 2xUNO. Today --
dated 20 March 2018, one UNO seems to be not being uploaded with sketches. The symptoms are:

1. Verify/compile OK!
2. During uploading, the TX/RX LEDs on the UNO are active; but, the uploading ends (terminates) with the following error message:

avrdude: verification error, first mismatch at byte 0x0000
** 0x62 != 0x0c**
avrdude: verification error; content mismatch
avrdude: verification error; content mismatch

3. I have replaced the ATmega328P (suspected one) from the 2nd working UNO.
4. Now, program codes are uploaded and everything is fine.
5. What is rong here -- ATmega328P or the Boot Loader or the code memory?
6. Would erasing the ATmega328P and re-loading the Boot Loader be helpful to get my chip back?

Try #6.

Thanks for the reply, @jremington!