I was Tabbing + Shift Tabbing in the Diagnoses Tab.

make object! [
code: 303
type: 'script
id: 'expect-arg
arg1: 'tail
arg2: 'series
arg3: [series! port!]
near: [view*/caret: tail face/text if face/para]
where: 'focus
]

The GUI is still a little fragile, and so rapid tabbing etc might cause problems.

That's exactly what happened. I was hammering the Tab / Shift Tab for some reason. It certainly wasn't "typical use".