When editing en.wikipedia, the shift key on the first keystroke causes the cursor to jump to the bottom.
Steps. Edit an article. Place cursor where you want to edit the text. If the first keystroke involves using a shifted character, like a capitalized letter at the beginning of a sentence or a common wiki character like { or |, it will instead cause the cursor to jump to the last position on the window and will write the keystrokes.
Solution, cut or delete the error and return to where you intended to work.
I edit using Safari v10.0.1 on a Mac OSC 10.12.1
This is similar to but distinct from the common wikipedia editing problem that has plagued me for almost ten years where, after placing the cursor, it will disappear if the mouse does not remain hovering over the edit window. In the case above, it does it even when the mouse is still hovering over the edit window. This has been happening for about a month. ~~~~