Since June 28th, the build settings now display a warning when a build command / publish directory / base directory is specified that differs from the one specified in netlify.toml:
Personally, I’ve been bitten by this multiple times, where I tried changing the build command in the UI, only to realise 20mins later that the netlify.toml was overriding it all the time, and my changes were ignored.
The new warning is based on the netlify.toml of the currently published deploy.