What happens if a taxpayer forgets to file ITR but wants a refund later?

@ Ayush

One can file belated ITR till 31st March of the Assessment Year following the Assessment year in which the ITR filing was due.

With penalty.

And refund could be claimed in the belated ITR.

For more delayed cases, I am not aware of any rules, except CBDT keeps on giving opportunities time to time - gives circulars. 

So daily spend five minutes checking CBDT circulars and you may strike silver if not gold.