PROBLEM
In the node editor, if the color in a swatch happens to match the background of the node (and this happens weirdly often (???)) the border around the color swatch is near invisible (on my system with my theme). This lead to confusion as to whether there was a swatch at all.
SOLUTION
Fortunately , this may be the easiest fix of all time: increasing the border size by one pixel would probably do it, or adjust the relation between the swatch outline and the background such that they are always maximally contrasty (XOR-type color).