How to monitor variables not using the serial?

Neat. I must try this.

DITTO.

Don't tell anyone I told you this but you could do a print from an interrupt routine

.So nobody knows who to blame when things go to hell?

I use Serial.flush() with an counter to empty the buffer if I think there is a chance of that happening.

Waits for the transmission of outgoing serial data to complete. (Prior to Arduino 1.0, this instead removed any buffered incoming serial data.)