Emotions is widely used and expresses feelings, mode and tone of the sender of the message. It’s mostly used in between people communication, where you in a text message can convey irony, sadness, excitement or happiness, which would take too long to write out, especially if the number of characters are limited.
In the more recent years, these emoticons have also been used in machine to person communication between (usually) a web application and a user. It’s often used when something unexpected happens; the web app governor doesn’t want to scare the user away. Throwing an extensive text message or even worse a 404 file not found message, makes users uncomfortable. Instead of explaining in text, you want the user to feel secure and assured that the web app owner are sorry for what happened as fast as they can. And the fastest way to do that is to use something the user is familiar with, a big sad face emoticon – and the user immediately know that it wasn’t them that caused the problem, it was the web apps fault.
A very smooth way to convey messages - use the language and symbols in the users domain. I can only applaud these emoticons in web apps. Well Done!
Wikipedia definition of Emoticon:
An emoticon is a meta communicative pictorial representation of a facial expression which in the absence of body language and prosody serves to draw a receiver's attention to the tenor or temper of a sender's nominal verbal communication, changing and improving its interpretation. It expresses - usually by means of punctuation marks - a person's feelings or mood and can include numbers and letters, as well. These emoticons are usually flipped sideways. In the most recent years, as social media has become widespread, emoticons have played a significant role in communication through technology. These emoticons offer another range of "tone" and feeling through texting that portrays specific emotions through facial gestures while in the midst of cyber communication.
The warning message
I don't know of any warning messages, but things you can try (and test on your target audience) are the following: