The Mathematics Education Reserach Blog has linked to an article by Mashooque Ali Samo titled Students’ Perceptions About the Symbols, Letters and Signs in Algebra and How Do These Affect Their Learning of Algebra.
I found this part interesting:
Falkner, Levi and Carpenter (1999) asked 145 American grade 6 students to solve the following problem:
8 + 4 = ___ + 5
All the students thought that either 12 or 17 should go into the box. The equal sign meant “carry out the operation”. They had not learned that the equal sign expresses a relationship between the numbers on each side of the equal sign. This is usually attributed to the fact that in the students’ experience, the equal sign always “comes at the end of an equation and only one number comes after it” (Falkner et. al., 1999, p. 3). Another possible origin of this misconception is the “=” button on many calculators, which always returns an answer.