I have a board based on an ATMega1284P. I am using a 20mhz xtal bypassed with 22pf caps. These boards have been in use for over 5 years (on for maybe 10 hours per week).
The other day the board was running when I put my scope probe on XTAL1 or XTAL2 (I do not recall which) and the board no longer runs - correctly. The scope was grounded to the signal GND on the board but the probe was on 1X rather than 10X.
I can connect with JTAG, load memory, read memory, read fuses, etc. but it will no longer run - correctly.
I say "correctly" because it appears to execute the bootloader. My (serial) bootloader turns on a status LED when it is invoked and turns it off when it exits to the application. The status led is coming on but never going off. During the 1 second the bootloader is accessing the on-board USART looking for a connect. If it does not receive a connect within 1 second it starts the application.
I'm about to go out to the shop and investigate some more but I wonder if anyone can shed any light before I resort to removing the (SMD) processor.
I'll post testing results later.