Steps to Reproduce: 1. Upload a sketch the Arduino that communicates via serial, the ASCII table from the examples works well for this, or a simple Serial.println("Hello World"); 2. Open the Serial Monitor windows 3. Unplug the USB cable from the Arduino to simulate tripping over the cord or other reason for communication loss. 4. Notice how the entire IDE is now useless!

Ideally, a message would be displayed, or even nothing at all would happen. However, the entire program freezes and the whole process must be killed.

What version of the Arduino software are you using? On what operating system? Which Arduino board are you using? This occurs with Arduino software 0017 (The latest version as of this reporting time) on Kubuntu Linux 9.04. I used an early Duemilanove, the one with the ATmega168.