Phase margin is usually defined as how far is the phase of the loop gain from -180: $$PM = \angle \text({loop \space gain}) +180^{\circ} $$
Let's suppose that we want to compute the maximum phase margin of a single-pole amplifier. For a non-inverting amplifier, this calculation is straightforward: PM(max) = -90 + 180 = 90. However, when it comes to inverting amplifiers, things can get tricky because of the initial 180 phase delay that already exists at low frequencies between the input and output.
For example, using the previous definition, one can compute PM(max) = (-180 - 90) + 180 = 90 or PM(max) = (+180 - 90) + 180 = 270. Thus, we get different answers depending on how we consider the initial phase delay (+180 or -180). However, intuition suggests that the correct value should be PM(max) = 90 because it is the amount of phase delay left until the input signal "inverts again".
What is right value of the phase margin in this case ?