A simple approach to the realisation of a differential Colpitts oscillator is to connect the grounded capacitor in the feedback network of two single-ended Colpitts oscillators together in series. As the output signals are expected to be differential, it is proved in this Letter that, this is not always the case. In certain circumstances, this topology exhibits a common-mode output signal, or even worse, a combination between differential-mode and common-mode waveforms. Design criteria for avoiding these undesirable oscillating behaviours are derived by considering the start-up characteristic of the oscillator.