It took me a while to figure out the differences between the AddHandler and AddType directives in an Apache configuration. Both directives seem to define how the contents of a file with a given extension should be threatened but when do you need the first one and when the second?
Strict versus quirks mode
Something slightly related is how browsers actually deal with the content type you set in your AddType directive. Gecko based browsers (like Firefox) follow the standard closely so they will handle a file according to the content type in your Apache configuration. This means your image will show up as text if you wrongly assigned the text/plain content type. Internet Explorer is more flexible and tries to guess the mime type for misconfigured web servers showing a wrongly defined images as, well, an image.