Splitting in blackjack can be a risky move, especially with certain card combinations. One of the worst cards to split is a pair of 10s, as it’s a strong hand that totals 20. Another unfavorable split is a pair of 5s, as they add up to a decent hand of 10. A pair of 4s is also considered one of the worst cards to split, as it’s better to play them as an 8. Additionally, a pair of 9s is generally advised not to split, as 18 is a strong total. Lastly, a pair of 6s is often recommended not to split, as they can easily turn into a bust hand.