This high amount of stress concentration can often lead to failure of plastic parts.
Sharp corners can come about in non-obvious places.
Examples of this are a boss attached to a surface, or a strengthening rib. These corners need to be radiused just like all other corners. The stress concentration factor varies with radius, for a given thickness. The stress concentration factor is a multiplier factor, it increases the stress. This is why it is recommended that inside radiuses be a minimum of 1 x thickness.