T193458 describes a situation where complex maps have begun failing. While we look for an answer to why this began happening recently. In the task, @Catrope makes the following suggestions:

• We need better error messages for this case. "Invalid GeoJSON" is not what really happened if we had a timeout looking up the shapes.
• Paul suggested looking at the GeoJSON that this lookup tries to generate. 200+ highways might correspond to megabytes of GeoJSON. That can be OK for static map generation, but won't be OK for in-browser dynamic maps where all that data needs to be downloaded to and rendered by the client.

I'm not sure exactly what error message will be required, but am happy to consult when someone can tell me exactly what the situation that triggers the message is.