Sharp corners greatly increase the stress concentration. 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.
As can be seen from the above chart, the stress concentration factor is quite high for R/T values lesss than 0.5. For values of R/T over 0.5 the stress concentration factor gets lower.
The stress concentration factor is a multiplier factor, it increases the stress.
Actual Stress = Stress Concentration Factor K x Stress Calculated
This is why it is recommended that inside radiuses be a minimum of 1 x thickness.