Sportscene CW not doing their jobs properly

My son bought him a pair of adidas slippers at CanalWalk Sportscene end November. He wore it for the last few weeks not realizing that it is mismatched sizes. When I paid for it on my account I also didn't realize that it was not the same size, which is also partly on me. However, when purchasing another pair of slippers yesterday, a consultant at Totalsports Zevenwacht Mall brought to my attention that it is the cashier's job to ensure the sizes and everything is correct, before we leave the store. I must say I am very disappointed that the staff at Sportscene CW would fail to do their job properly. I am definitely not shopping there again.