I noticed last night while adding some cards to my collection that the site has some issues with apostrophes within names. Charlie O'Brien, Paul O'Neill, Troy O'Leary, etc - I was adding multiple cards of these players, so I clicked the "update your collection" link on their personal page.
I could get to the update page, but when I clicked a checkbox it wouldn't actually save to my collection and the row wouldn't turn blue.
I ended up having to click through to each individual card's page and check those off - not a huge deal, but certainly not convenient.
I didn't try this, but the cards may add if you hovered over the drop-down and clicked "add another" - but then you'd constantly have to reload the page.