Gmail's contextual error prevention

There are many obvious reasons why people enjoy Gmail: it blocks your spam, you get bundles of space, and it keeps things neat and tidy. But sometimes it's the subtitles that make it truly enjoyable. 

One thing everybody does, which is followed immediately by a facepalm, is forget to include the essential part of your email: the attachment. The following “Oops, here's the attachment” is an unpleasant experience we could do without. 

But thankfully for Gmailers, this hiccup has become something of the past.

Why this is really good UX:

• One of Jakob Nielsen's Usability Heuristics stresses the importance of communicating errors gracefully and clearly. But what's even better—as Gmail does here—is preventing the user making a mistake in the first place.
• The error message is contextually aware. It only fires under the specific conditions of: a mention of an attachment in the copy, the attachment being absent, and the action of pressing send. This message appears precisely in the right place at the right time.
• When the user mistakenly hits “OK” without an attachment—or has sent something they maybe shouldn't have—Gmail gives them one last lifeline. In the bottom left, the user has five seconds to hit “undo.” This can be upped to 30 seconds for the more indecisive user.