This is one of the eggcorns that show how word choice can slide under the pressure of what could be called the metaphoric potential of a word. For _sphere_ and _spear_, these are quite different: a sphere is a container, homogeneous, round, can grow or shrink; a spear is long and pointed, a weapon.
The substitution _sphere»spear_ can occur as a spell checking artifact, too. But none the less it looks like a genuine eggcorn for at least some writers.