A perennial tweet by Jared Spool:
The default Mail application in iOS — perhaps one of the most popular apps in the world — is an unfortunate recent example. With the latest release of the app (in iOS 13,) Apple’s designers changed the options in the toolbar so that the Trash button is located where the Reply button has been in previous releases. Even though these buttons are different, app users have developed muscle memory over time; they’re used to tapping on that screen location to initiate a reply action without thinking about it. Only that now, instead of opening a draft reply, the message they’re working on disappears. The result? Frustrated users. John Gruber has a good overview of the issue and user reactions.
Few changes are as impactful as those you make to your system’s navigation elements. Toolbars and navigation bars are how users move around and do things in your app or website. Over time, people get used to where options are; changing their placement — even if done for good reasons — can lead to frustration. If you must change long-established navigation elements, test new versions extensively with users of varying degrees of experience. And if you don’t have good reasons to change your navigation elements, consider focusing on other aspects of your system instead.