The Blinovitch Limitation Effect was the effect or non-effect of crossing timelines.
The effect was named after Aaron Blinovitch who formulated the Blinovitch theory in a reading room in the British Museum in 1928 on Earth. (MA: The Ghosts of N-Space)
Theory A
The Limitation Effect limits the amount a person can change his time line, in fact any changes which deviate create a time loop. (DW: Day of the Daleks)
Theory B
The effect is the energy released when crossing one's own timeline and interacting with a past self (or future self depending on perspective) and the energy thus released. It causes memories to be transferred from the past self to the future self, causing the person to suffer amnesia until he completes the interaction from the other side. (DW: Mawdryn Undead) This aspect of the effect, however, does not appear to come into effect when different incarnations of a single Time Lord, such as The Doctor, interact with each other (DW: The Three Doctors, The Five Doctors, et al.), or when different versions of the same incarnation interact (DW: The Big Bang).
Theory C
The Limitation Effect stops a time traveller from messing around with his own personal timeline. The limitation effect is a precursor to a paradox (probably). (DW: Day of the Daleks)