I see the following message in the console quite often.
[16855:16855:1015/102718:ERROR:CONSOLE(154)] "Execution context with given id not found.", source: chrome-devtools://devtools/RuntimeModel.js (154)
I suspect it has something to do with the recent changes in the execution context management on the front-end.
Closing as invalid, as this bug pertains to the old inspector UI and/or its tests.
Please file a new bug (https://www.webkit.org/new-inspector-bug) if the bug/feature/issue is still relevant to WebKit trunk.