WordPress
Plugin conflict
Plugin conflict is when two WordPress plugins, or a plugin and the theme, break each other when they run on the same request. The symptoms range from a fatal error to a button that silently does nothing.
How it is measured
You confirm one by subtraction. Deactivate everything but the suspect, switch to a default theme such as Twenty Twenty-Five, and reproduce the bug. Then reactivate one at a time. The Health Check & Troubleshooting plugin does this in a private session so visitors see the normal site, and wp plugin deactivate --all does it from the shell.
Evidence comes from two places: the PHP error in wp-content/debug.log, such as Cannot redeclare function foo_init(), and the browser console, where an Uncaught TypeError points at a script handle from one of the plugins.
Worked example
A booking site's Pay Now button stops working the morning after a form plugin updates. The console shows $ is not a function on the checkout page. Query Monitor lists two jQuery copies, 1.12 and 3.7, because a slider plugin enqueues its own older jQuery under a different handle.
The developer dequeues the slider's copy on the checkout page only. The button works again, and the slider keeps running on the homepage.
How it differs
A plugin conflict is a failure between components, while a single plugin can also just be buggy on its own. A must-use plugin that sets a constant or a filter can cause a conflict that the Plugins screen will not show you, because you cannot deactivate it there. A conflict also differs from a server error, which would happen with every plugin off.
Common errors
Updating five plugins at once and not knowing which one caused the break. Testing with the cache on and reading a stale page. Skipping the theme in the test. Blaming the newest plugin, when the older one changed its behavior. Debugging on production for visitors rather than on staging.
In practice
Reproduce on a staging copy. Use the Health Check troubleshooting mode to bisect plugins, check mu-plugins by hand, and copy the first error from debug.log into the bug report. Update plugins one at a time and test the key flow after each.