Gravity is a secondary effect of electromagnetism! A magnet that attracts gold!

[Verpies]

I wonder how common this mistake is.

Unfortunately it is very common. Fortunately, in most usage cases it makes a small difference.
The high-frequency current jumping turns and layers is very common, too ...unfortunately, this one makes a huge difference in most HF usage cases.

This is aggravated by the ubiquitous ignorance of the Fourier transform, e.g.: 30 pulses per second that rise or fall in microseconds or nanoseconds contain HF components in the MHz range.