What Are the Defects That Generally Occur in Injection Moulded Items?

There are 3 categories of defect.
1. Caused by the process
2. By the tool itself
3. By the design of the part

examples..

1. Mold not filling completely due to plastic too cold, insufficient pressure, insufficient time .
2. Mold not filling completely due to poor design of feeds, vents, cavity shape etc
3. problems with fit and finish due to poor design of the actual part without reference to the actual process or material. You often find 'designers' who come up with stuff that looks great but can't be manufactured cost effectively
You can apply the same 1,2,3 thought process to most defects and problems.
I think a visit to a mold shop a chat to an operator and a rummage through the reject bin would help you.
Also take any cheap plastic item and take it to bits, look for the ejector marks, flash and witness lines where sliding cores operate. Also marvel at the awful design of most plastic clips... (the long thin ones that snap off at the root because they are not blended in or tapered)
Other than that, I have no idea.
Del