Now when you use the Gmail mobile web app, you will have a small window of opportunity to undo four key actions: archive, delete, add or remove a label, or move a message/conversation. When you take one of these actions, Gmail displays a yellow bar that recaps what you just did and allows you to undo it:
This bar stays in position even if you move to another screen (e.g. moving to "Menu" from "Inbox"). If you do not happen to catch your mistake in time, not to worry: all four actions can still be undone through other means (e.g. you can move a message from Trash back into your Inbox).
Try it out at gmail.com in the browser of your Android or iOS device.
[Source: Gmail blog]