Neutral Shift

no...

It can, if it becomes ungrounded.

For example, in a +120/neutral/-120 system, unequal loads on the + and - sides will will cause the + and - voltage to shift (still totaling 240).

What you said is a 2 phase supply, the OP asked about single phase system.

Dead wrong; what I described is single phase, opposite polarity. (There have been some 2-phase schemes separated by 90°, but they are pretty oddball.)

+120V/ Neutral / -120V is exactly a 2 Phase system (180 deg apart) .

2 phase system separated by 90 deg is unheard of, where is it used?

Baloney. +120/N/-120 is not defined as 2-phase; 180° apart is just the same phase, opposite polarity.

I already said that 90° separation is a bit of an oddball [historical, maybe]. Just because you haven't heard of it does not make it unheard of in general.

Check out www.3phasepower.org/2phasesystems.htm. I'm certain I first read about it on CR4 several months ago. The question had to do with lighting circuits.

Scott transformers, I've never seen one but they exist

http://en.wikipedia.org/wiki/Scott-T_transformer

Next you'l be saying a 48 phase transformer cant exist. But they were used for murcury arc rectifiers.

What Tornado and you are discussing is called a split phase system of only one phase. This is often erroneously called a 2 phase system because of the semantics of the English language but it is only one phase.

This is a very common mistake to make. I've made it here myself.