Cypress - first test randomly fails with "Invalid or unexpected token" - cypress

Question

Recently switched to using Cypress parallel for our Angular project in our pipeline. We run on a Codebuild on AWS and run 5 threads of the Cypress runner. About a quarter of the time, the first test on one of the threads fails with this error:
An uncaught error was detected outside of a test
Invalid or unexpected token
This error originated from your test code, not from Cypress.
When Cypress detects uncaught errors originating from your test code it will automatically fail the current test.
Cypress could not associate this error to any specific test. We dynamically generated a new test to display this failure.
Tried many things to try to fix this, including setting modifyObtrusiveCode to false, chromeWebSecurity to false, upgrading Cypress. We are already catching uncaught exceptions so that doesn't seem like it should be the issue. I turned on some extra logs for this and here is the output
[3] 2020-03-06T19:57:20.369Z cypress:server:project onMocha start
[3] 2020-03-06T19:57:20.369Z cypress:server:reporter got mocha event 'start' with args: [ { start: '2020-03-06T19:57:20.366Z' } ]
[3] 2020-03-06T19:57:20.374Z cypress:server:project onMocha suite
[3] 2020-03-06T19:57:20.374Z cypress:server:reporter got mocha event 'suite' with args: [ { id: 'r1', title: '', root: true, type: 'suite', file: 'cypress/integration/ci-tests/content-acquisition/channels/channel-manual-upload-run-acquired-items-tab.spec.ts' } ]
[3]
[3] 2020-03-06T19:57:20.390Z cypress:server:project onMocha test
[3] 2020-03-06T19:57:20.391Z cypress:server:reporter got mocha event 'test' with args: [ { id: 'r2', title: 'An uncaught error was detected outside of a test', body: 'function throwErr() {\n throw err;\n }', type: 'test' } ]
[3] 2020-03-06T19:57:20.555Z cypress:server:reporter got mocha event 'fail' with args: [ { id: 'r2', title: 'An uncaught error was detected outside of a test', err: { message: 'Unexpected end of input\n' + '\n' + 'This error originated from your test code, not from Cypress.\n' + '\n' + 'When Cypress detects uncaught errors originating from your test code it will automatically fail the current test.\n' + '\n' + 'Cypress could not associate this error to any specific test.\n' + '\n' + 'We dynamically generated a new test to display this failure.', name: 'Uncaught SyntaxError', stack: 'Uncaught SyntaxError: Unexpected end of input\n' + '\n' + 'This error originated from your test code, not from Cypress.\n' + '\n' + 'When Cypress detects uncaught errors originating from your test code it will automatically fail the current test.\n' + '\n' + 'Cypress could not associate this error to any specific test.\n' + '\n' + 'We dynamically generated a new test to display this failure.' }, state: 'failed', body: 'function throwErr() {\n throw err;\n }', type: 'test', duration: 179, wallClockStartedAt: '2020-03-06T19:57:20.374Z', timings: { lifecycle: 26, test: [Object] } } ]
I couldn't really make anything of these errors, but maybe someone else can. I'm kind of out of ideas on what to try (I've tried more things today than I've listed but can't recall them all). Any ideas?

Answer 1

as setting modifyObtrusiveCode to false didn't help you as the folks in https://github.com/cypress-io/cypress/issues/6132 .. I can give my debug procedure when I encountered a similar flakey "unexpected .." error with Cypress:
cypress run has a burn= param, able to repeatedly run. Enable .har output recording for those runs with the cypress-har-generator plugin.
When you have two groups of successful and failing example .har files for the same request, open them in a Browser to compare if anything stands out.
I used diff + jq queries on the .har files to compare between the groups content per request path, but already opening a failing .har in the browser inspector network tab showed a 30s processing time for a .js path that was ultimately incomplete, and thus violated js syntax, causing an unexpected end of input error, similar to your "unexpected token".
Interestingly this occured to the same file at the same code line, hinting at a parsing problem in Cypress.
We exchanged that dependency (or specifically - updated it and changed how it was webpacked) and Cypress stopped to hiccup on the ressource, the flakiness disappeared.
My impression is, running parallel threads of Cypress contributes to the problem occuring.