Attempting to use an unsafe value in a safe context.
The value provided for use in a specific context was not found to be safe/trusted for use.
Angular's Strict Contextual Escaping (SCE) mode (enabled by default), requires bindings in certain contexts to result in a value that is trusted as safe for use in such a context. (e.g. loading an Angular template from a URL requires that the URL is one considered safe for loading resources.)
This helps prevent XSS and other security issues. Read more at Strict Contextual Escaping (SCE)