What's happening
WordPress tried to modify HTTP headers after content had already started flowing to the browser. In production this usually traces back to plugin conflicts, accidental PHP output, misconfigured caching, or production debug flags that should have been turned off long ago.
Warning: Cannot modify header information - headers already sent by...
output started at /wp-includes/functions.php
Why it matters
Some companies only learn about this error when their own customers start reporting it. By then the site has been quietly exposing internal paths, PHP traces and stack details to every visitor for hours — or weeks. And it's rarely the only operational symptom.
Technical exposure Internal paths, file names and PHP traces visible to every visitor.
Continuity Redirects, sessions and forms can silently break with no clear signal.
Trust Visitors read it as neglect, even when the business itself is fine.
Common patterns
This warning rarely appears alone. It usually co-occurs with outdated versions, half-finished configurations, abandoned plugins, and environments where development and production aren't cleanly separated.
plugin conflictwp_debug enabledcache misconfigurationbroken deploymentoutdated php