When typing in visual editor, sometimes pressing CTRL+Z will replace the last entered character with a pawn (♙).
I've encountered this most frequently when undoing text that appeared as a link when I didn't want it to (see bug 51531) but occasionally when undoing other text as well.
There are numerous other pawn insertion bugs, other editors have suggested possible relationships with bug 49732, bug 48346 or bug 51140, but none seem quite the same (e.g. there is no non-ASCII text, bold or italics involved) and all are marked as fixed. I encountered this yesterday and today (16 and 17 July).