Phrase most notably used by teachers as a replacement for "I was incorrect," or "I presented the wrong information (on purpose)."
The usage of this phrase is likely a result in the instructor's attempt to be make witty statements; however no extra attention is given to "I lied" over "My mistake" or "This new concept renders the old one false."
Origin unknown. The phrase may have started with teenagers who felt the need to express faults in a comical manner. It seems that the trend ended, and teenagers have reverted to "just kidding," which has been reduced to "j.k. j.k." Middle school and high school instructors must have picked up the trend upon hearing several students use the term.
See: jk, j/k, jp
Teacher: I told you guys that the normal force is always opposite of the force of gravity. Well, I lied, because now we are moving into forces and inclines.
Teacher: I lied, the pop quiz will be added into the homework category.
Teacher: Oops, I lied. The color yellow does hold a significant value in the book Crime and Punishment.