All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
If such a sign happened unintentionally and that person did not intend to make the sign of the cross, then he is not sinful.
In case he intended to make the sign of the cross, then he is imitating the non-Muslims and this is a forbidden matter. The Prophet said: “Whoever imitates a people is one of them.” [Ahmad and Abu Daawood]
However, in case he intended to glorify the cross by doing so, then this is apostasy as the scholars defined apostasy by: “A Muslim becomes an apostate by a clear statement of disbelief or a wording or an act that implies it, like throwing the Mus’haf (the Arabic text of the Quran) in the dirt, and wearing a Zunnaar (a wide cloth belt worn by priests around their waists).”
Finally, it should be noted that if a Muslim jokes, he should only say the truth as the Prophet did when joking; for more benefit, please refer to Fatwa 82375.
Besides, Sharee’ah matters should not be a subject for joking and mocking as this may lead the person to Kufr [disbelief]; for more benefit, please refer to Fatwa 26193.
Allaah Knows best.