Playhead won't stop at the end in Final Cut Pro on iMac

Kristen, that's not supposed to happen, but over the years I've seen in a few times. One possible cause is having a collapsed audio lane that invisibly extends beyond your expected timeline end point. To check for that click on any audio or A/V clips toward the end of the timeline and do Clip>Expand Audio. If you have Compound Clips, select those and do SHIFT+CMD+G which will expand them. You can revert that with CMD+Z. If those clips themselves have audio, expand that as above.

If none of those steps show an audio lane extending to the end of the timeline, as Ian said, reset FCP settings. See this page for specifics: Change settings in Final Cut Pro for Mac - Apple Support.

In general FCP has excellent reliability and data integrity, but there have been a few rare cases where a damaged project database can cause this kind of behavior. One way to solve it is exporting a project XML, then re-import that to a new fresh library. Examine that timeline and see if the problem has been fixed.

Was this a one-off event or can it be repeated? Send a screenshot of the timeline.

If it occurs frequently delete the preferences by launching FCP with the alt and command keys pressed down until a window appears.

Click "Delete" in the window that opens.

I don't see it as much as I used to. It was a bug several versions back that was supposed to be fixed. But every blue moon I still see it happen. Not much you can do. And it can happen when there's absolutely nothing to be included that far down the Timeline. I think I've noticed it's when I had media that far along, and then deleted it. A relaunch or two of FCP and it starts to work fine, again.