The worst single-character variable overloading has to be the computer engineers building robots and such.
i, j, and k are common variable names for loops. EG "For i = 1 to 100" etc.
i, j, and k are unit vectors for the X, Y, and Z axes. The mechanics of your robot or other physical system will use this.
I is current.
i is the imaginary unit for mathematicians, and j is the imaginary unit for EE/CEs.
So i can have four possible meanings (possibly more) depending on case/font/vector notation, j can have three, and k two. Context becomes very important.