My remote will launch a different app when I try to resume watching TV after pausing. I'm not 100% positive about this, but I think it has only happened when the television program has been paused long enough for the screensaver to pop up.
Some screensavers, including Roku's Movie Magic, Skyline, and others include ads (or at least links) that can be launched from the Play or OK buttons on the remote.
If you have a screen saver that has ads/links, and use one of those buttons, you're launching the ad, which is usually to an app you may or may not have installed.
Use the Back (not Left Arrow, but the Back button) or Home to exit a screen saver.
DBDukesRoku Community Streaming ExpertNote: I am not a Roku employee.
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Switch the screensaver to the clock.