Find Component Markup Errors Faster with Improved Error Messages

We’ve improved the details provided and the presentation of run-time errors caused by certain kinds of Lightning component markup errors. It’s now easier for your users to report errors they encounter, and for you to find and fix those errors. This change affects all users of your org, and doesn’t depend on debug mode being enabled.
Because a picture is worth a thousand words, here are two of them.
Spring ’17 Error Message Previous Release
Component markup error in Spring '17 Component markup error in earlier releases
Note

Note

The new error panel doesn’t have an OK button. To dismiss the panel, click the “X” in the top right.

The new error panel appears only when a relevant error occurs in Lightning Experience or Salesforce1. The error panel’s expandable stack trace appears only on desktop and tablet form factors—not on a phone.

Only certain kinds of markup errors result in the new error reporting. Errors in your component controller or helper JavaScript code display with the old error output. The new error reporting affects only your custom components, not standard components provided with the framework. Outside of Lightning Experience and Salesforce1, for example, in Lightning Out or a standalone app, error output can include details that were previously omitted, such as a complete stack trace.