Electrical crosstalk
Headphones with an unbalanced electrical connection commonly use a three-wire cable in which left and right channels share the earth conductor. The result of this mutual earth impedance is inter-channel crosstalk, which is frequency-dependent in level if the headphone has frequency-dependent electrical impedance. This graph shows the severity of this effect by measuring the relative level of electrical crosstalk from the left to right capsule.

Electrical crosstalk is extracted using an isolator box connected in series with the headphone which open-circuits the right channel feed to allow the crosstalk signal from the left channel to be measured:

The signal at terminal A is the input signal; that at point B is the electrical crosstalk signal. Academic research (reference below) suggests that only when crosstalk reaches -15dB does it compromise stereo imaging, but a safety margin of a further 10dB at least (ie -25dB or better) is easily achievable.

Measurement parameters
Test signal:                                                       pink-spectrum periodic noise
Sampling frequency:                                      96kHz
Bit-depth:                                                         16-bit
FFT length:                                                      32,768pt
Frequency resolution (measurement):       2.93Hz
Frequency resolution (graph):                      1/100th octave
Measurement averages:                               10
Trace smoothing:                                           none

J M Adkins and R D Sorkin, "Effect of Channel Separation on Earphone-Presented Tones, Noise, and Stereophonic Material", Journal of the Audio Engineering Society, v33 no4, April 1985 (available here)