What happens if a player has 5 cards and forgets to exchange them in Risk?
There are cases where cards can be turned in during a turn; for example, if conquering another player results in the victor having more than four cards. In this case, the rules say the cards should be redeemed immediately, the new armies placed, and the turn continued. 

I think if someone has started their turn and failed to redeem cards they were required by rule to redeem they should have to do so immediately upon someone noticing. This could provide an advantage because they’ll have the knowledge of how a couple if rounds went, but often armies are all stacked in a giant group. 

If they finish their turn and no one notices,  when they do ultimately redeem I say they should get the number of armies they were originally entitled to and not a larger number—which would be the case if other players have redeemed since then. The reason is that sandbagging turn ins to get higher army counts is why turn-in at five cards is required in the first place.