I had my Nexus 5 updated to Android 5.0 recently. I notice the following "interesting" behavior of the new notification system in Android 5.0.
As you know, when there is an upcoming alarm the stock Alarm app displays an advance notification, offering the opportunity to dismiss the alarm before it happens. The notification is a rectangle, saying "Upcoming alarm" and stating the next alarm time and name. It normally has a "DISMISS NOW" link at the bottom.
The stock Calendar application also displays Android notifications for upcoming evens, if an advance pop-up notification is requested in event settings.
However, if the Alarm notification comes up together with a Calendar notification (i.e. they are stacked one above the other in the notification screen), the "DISMISS NOW" option is always missing from Alarm notification. The upcoming alarm notification looks as usual (a rectangle with future alarm time and name), but there's no "DISMISS NOW" link at the bottom.
Calendar is the only app I discovered so far, whose notifications have this effect on Alarm notifications. For example, GMail notifications can co-exist with Alarm notifications: they do not make the "DISMISS NOW" option to disappear from Alarm notifications.
So, what exactly is the idea behind this strange user interface behavior? Is this a bug? Or is this by design? Is there a trick for dismissing upcoming alarms in a situation when the Alarm notification shows no "DISMISS NOW" option?