This is not a bug per se, but the display is a little confusing. The important tiny detail to note is that, although the RuntimeError is still displayed, the left border has gone from red back to gray indicating that the cell is computing.
So, this is a syntax error:
And this is a cell that is computing (that previously errored):
The old error is still displayed because Observable continues to display the previous value of a cell until the new value resolves. And in this notebook, that value won’t resolve until you type in a notebook’s slug in the top input and click “Go”, so you could be waiting a while.