I worked on the communication middleware for an early Windows tablet. My part sat between a VB6 GUI app (that we customized for each customer) and a RS-232 device and handled all the communications.
One customer had intermittent communication failures that we couldn't identify for weeks. I finally went on-site and hooked up a serial port logger between the tablet and device. After an hour of testing, I finally captured what caused the communication failure. It was some debug message from the GUI app, somehow coming across the serial port. I called back to the office and had a guy from our group go over the the VB app team and ask which genius was logging stuff to the serial port. Sure enough, some guy had been using VB's remote debugging feature with a null modem cable and left it turned on in the shipped app.
One customer had intermittent communication failures that we couldn't identify for weeks. I finally went on-site and hooked up a serial port logger between the tablet and device. After an hour of testing, I finally captured what caused the communication failure. It was some debug message from the GUI app, somehow coming across the serial port. I called back to the office and had a guy from our group go over the the VB app team and ask which genius was logging stuff to the serial port. Sure enough, some guy had been using VB's remote debugging feature with a null modem cable and left it turned on in the shipped app.