Nodes are sorted based on the selection. In some cases (even depending
on processor speed, nodes can be selected and reordered, and another
operation can run before the next redraw). That gives a window where
operators mapped to the same input as selection can run with invalid
socket locations (which aren't updated after the nodes are reordered,
since they are stored in a separate array).
To fix this, move the socket locations from the node editor runtime
data to the node tree, tag them as invalid when the nodes are
reordere, and check for that status in a few more places.
A better longer term solution is not reordering nodes based on
UI status and instead storing the UI drawing order separately.
For reasons described in more depth in the new code comment,
reordering nodes can sometimes invalidate the socket locations
in a way that makes random connections after a node is selected.
A better change would probably be to stop reordering nodes and
store their "UI order" separately instead, but that's more involved.
On its own, storing socket locations in a SoA format probably isn't
worth this complexity, but I think it's useful to have something like
this working so more data can be stored this way in the future.
I think the reproduce-ability of this bug depends on Blender's
performance, since I had a much easier time recreating it in a
debug build with a bunch of extra nodes added to the file.
Hans Goudey