Often, a broken connection between a circuit and oscilloscope probe can act as a tiny capacitor. This capacitance combines with the probe's input resistance to form a differentiator.
Note: this circuit includes a relay (activated by pulses) to simulate a broken connection between the signal source (left) and oscilloscope (right).
Rather than showing the square wave, instead the scope shows a differentiated signal with voltage peaks and valleys at the edges of the square wave (when voltage is changing rapidly).
