Errors messages are a tricky part of the HMI design. An error message means that the user cannot reach the goal of an interaction and it therewith carries a negative emotion. However, the adherence of simple principles can improve the design of an error message and consequently brighten the user's mood. An error message should be simple, easy to understand, easy to locate, and, in the best case, instructive. Simplicity means that the message should be as short as possible and as long as necessary. An easy understandable message is communicated in a language that the user can understand, for example, avoiding technical terms and abbreviations, and clearly states a reason. Easy to locate means that the user can associate the message with the dedicated part of the interface. At best (and if adequate), the message should include instructions for the user. For example, that could be contact information if a service cannot be accessed. The messages below include an additional humorous element which further reduces potential negative emotions triggered by an error message.
Website content changes and links that a user saved as bookmarks are no longer valid. When the user then tries to open such an outdated bookmark, some sort of message must be communicated from the website to the user. There is a better design than stating "Website not found" or the display of a cryptic error code. The website Wetransfer uses pets to take responsibility for the message in a humorous way (image below). All those error messages take guilt away from the user, either in form of an apology for the error or in form of a funny note. There is a whole little series of thoughtful messages. Instead of being frustrated, one might be tempted to repeat the interaction to find yet another version of the error message.
Another example is the message from Duolingo. They employ their mascot to deliver the message. Comparing the design to the ideal, an improvement could be to change the title from "Error 404" to "Website not found". Positively, the message includes a descriptive text in easy to understand language and an instruction.
An example in a similar humorous design with a mascot is that from the online game website Kongregate