Hi Giles,

I have split the question into a new thread because it is important to keep each discussion with a clear subject making it easier for people to find solutions. The other thread was more of a question from a newbie about how to debug.

There are a few things to consider.

Normally break points on serial.print messages are okay but many users don't know how often their code is executing so it's possible to end up with many serial messages all on top of each other without any sort of gap/delay/spacing. For this reason best practice is for people to consider the volume of serial messages being sent or to allow gaps and time for the outbound messages to be sent.

The capability of the Arduino is affected more by the rate of hard coded serial.print() messages, number of debugger messages and speed of the serial. The user in the other post was using 9600 baud which is slow.

The arduino has to do a lot of work to transmit the messages. If the small Arduino serial buffer fills up to quickly it will wrap and cause previous outbound data to be destroyed. This should be a consideration for all Arduino projects regardless of debugger.

The pc has to do quite a lot of work to split up the inbound serial and display debugger data. If too many messages come in too quickly the pc won't cope and will fall behind in it's display of debugger status. 

So putting a debug message on a hard coded serial message should be avoid but if the break point is hit infrequently (ie: not in the loop()) it will be okay.