Release Incident Playbook

Critical path broken in one browser: symptoms, cause, and the test that catches it

A core flow fails in one browser or version after a build or polyfill change.

Critical path broken in one browser is documented by 2 accepted public reports from 2 organisations, 2 of them published in the last 5 years.

  • Likelihood: low (2 public reports in 5 years)
  • Impact: medium (2 of 2 reports: availability)

2 public reports · verified 2026-10-22

Symptoms

User-visible

  • After a minor server update, the web interface stopped while loading in the default iOS browser below version 16.4 and showed only the background picture.[1]
  • Clearing cookies or site data did not restore the web interface on the affected iOS browser versions.[1]
  • Downgrading the server to the previous patch release restored access from the affected iOS browser versions.[1]
  • The Files page of a file transfer service's web interface did not load in one browser, while other browsers were fully functional.[2]

Monitoring-visible

  • A user report in the public issue tracker raised the iOS loading failure, and a contributor reproduced it on an iOS 16.2 simulator.[1]
  • Customer support tickets about the Files page started at 4:00 AM PST, and support reproduced the error before an incident was called at 7:14 AM PST.[2]

Root causes

  1. stage: code A markdown parsing sub-dependency chain of the server's UI component library used a regular expression that the iOS browser below 16.4 does not support.[1]
  2. stage: code The unsupported regular expression was likely pulled in when a pull request started using the library's rich text component in the files app.[1]
  3. stage: code A deployment of the file transfer service included a software bug that affected only one browser.[2]

Public examples

  1. [1]Nextcloud, 2025-03-16. After Nextcloud Server was updated from 31.0.0 to 31.0.1, the web interface no longer loaded in Safari on iOS versions below 16.4 because a frontend dependency used a regular expression those versions do not support. primary [Bug]: safari on ios16 can't access web ui after update to 31.0.1 from 31.0.0, downgrade will resume ok
  2. [2]Files.com, 2022-09-06. A Files.com deployment included a software bug that affected only the Safari browser, so the Files page of the web interface did not load in Safari while other browsers worked, until the deployment was rolled back. primary Issue Accessing Files Page When Connecting with Safari

Test classes that catch it

Test classStageLimitationBasis
Compatibility testspre-deployA compatibility run on the current iOS browser passes, because the unsupported regular expression broke only versions below 16.4.by design analysis
End-to-end testspre-deployAn end-to-end test of the Files page run in any browser except the affected one passes, because the other browsers stayed fully functional.by design analysis
Synthetic monitoringpost-deployA synthetic check scripted in one browser never loads the Files page in the affected browser, and customer support tickets were the first signal of the failure.by design analysis

Risk

  • Likelihood: low (2 public reports in 5 years)
  • Impact: medium (2 of 2 reports: availability)
Counts behind the grades
Accepted public reports2
Published in the last 5 years2
Impact stated: availability2

How the grades are computed

Pre-release checklist

  • PR-01
  • PR-02
  • PR-03
  • PR-04
  • PR-05
  • PR-06
  • PR-07

First 30 minutes

  • FM-01
  • FM-02
  • FM-03
  • FM-04
  • FM-05
  • FM-06
  • FM-07

Download the checklist

What catches this: catalogue entries

Listed because their own documentation or site names compatibility tests, end-to-end tests, synthetic monitoring on the verification date. Alphabetical. Not a recommendation.

Tools whose documentation names these test classes

Providers whose site names these test classes

Only on this page

In report [1], critical path broken in one browser came from a sub-dependency, not from a change to the page that failed [1]. A markdown parsing chain inside the UI component library used a regular expression that the iOS browser below 16.4 does not support [1]. The web interface stopped at the background picture on those versions only [1]. A contributor needed an iOS 16.2 simulator to reproduce it [1]. A compatibility matrix that holds only current browser versions passes this release.

In report [2], support tickets started at 4:00 AM PST, and the incident was called at 7:14 AM PST [2]. Other browsers were fully functional throughout [2]. The rollback returned full functionality at 7:58 AM PST [2]. A corrected build was tested in staging and deployed at 9:26 AM PST [2].

Questions

What causes critical path broken in one browser?

In report [1], a sub-dependency of a UI component library used a regular expression that the iOS browser below 16.4 does not support. In report [2], a deployment included a software bug that affected only one browser.

What are the symptoms of critical path broken in one browser?

The web interface stopped while loading and showed only the background picture [1]. The Files page did not load in one browser while other browsers were fully functional [2].

Why did tests not catch critical path broken in one browser?

Neither report describes the tests that ran. The failure in report [1] appeared only on iOS browser versions below 16.4, so a run on current versions passes.

Which test class catches critical path broken in one browser, and what does it miss?

Compatibility tests, end-to-end tests and synthetic monitoring are listed, all with the by-design label. An end-to-end test run in any browser except the affected one passes, because the others stayed fully functional [2].

Which public reports document critical path broken in one browser?

Two primary reports from two organisations document it. They were published on 2022-09-06 [2] and 2025-03-16 [1].

Should a team roll back or fix forward when critical path broken in one browser happens?

Report [2] rolled back the deployment and restored full functionality at 7:58 AM PST, then deployed a corrected build at 9:26 AM PST. In report [1], downgrading to the previous patch release restored access.

Release Incident Playbook (2026). Critical path broken in one browser: symptoms, cause, and the test that catches it, content version 1.0.0. https://releasefailures.com/incidents/critical-path-broken-in-one-browser/. Accessed 2026-10-21.