PeterH:
AWOL:
Known feature.
Of course, it begs the question - why would you want to print more than one exclamation mark?
Or indeed, except for ellipsis, any more than one of any particular punctuation mark consecutively.To be honest, it doesn't beg those questions at all. The question it begs is why the designer chose to make the bootloader incapable of dealing with an arbitrary string, without either documenting it in any way that typical users could be expected to notice, or providing any means to warn developers who inadvertently used that string in their code.
It was a feature explicitly added by the Mega bootloader developer to 'trigger' a monitor feature in the bootloader code. This feature is probably not used by nearly anyone (other then the developer maybe? 
), but none the less the author needed to use some escape code to activate the monitor feature and therefore choose !!!. It was documented by the author of the bootloader, but the Arduino team did not do a very good job communicating it to the general population when it released the original mega board. I believe they have a new bootloader code ready (which also will handle WDT resets correctly) for the mega board but have not yet started to distribute it with the current IDE release and I have no idea if present mega boards are shipping with the new bootloader installed, but I bet no. Such is the case with a platform that relies on so many open source libraries and code modules not written by the platform developers. The problem was not with the original bootloader code developer but the Arduino team failure to properly communicate the 'feature' and their very slow process of implementing an updated bootloader for the mega board, even though it's been known and available for well over a year I believe.
Lefty