Long time lurker. I decided to make an account just to complain about this.
During checkout for Topman it allows you to apply multiple discount codes and gives you a green tick when they apply even if you apply multiple. What it doesn't display is that the last discount code is the only one that actually applies to the price.
I complained to their Customer Service and was told that only one discount can be applied, as if this is a well known fact even though it doesn't state this anywhere on the website.
Also with Student Discount they said that it can't be applied to another discount code. However if you read the T&C's regarding Student Discount this fact is not stated anywhere:
Let me know if this is against the rules somehow, but I thought it was a fairly useful info before you load up on a big cart and then realise you're not saving as much as you planned.