Someone claims that if a person laughs at a fault or misfortune of another person – for example a disability – then the person who laughs will have the same misfortune befall him as a punishment. Is this true?
We do not know of any evidence to support the claim that the person who laughs at another person's misfortune will in turn be beset with precisely the same misfortune.
This is a claim claim that gives a prediction regarding a person's future fate, which is a matter of the unseen. Therefore, such a claim can only be established with direct evidence from the Qur'ân and authentic Sunnah.
It is certainly sinful to laugh at another's misfortune. Allah says:
“O ye who believe! Let not some men among you laugh at others: It may be that the (latter) are better than the (former): Nor let some women laugh at others: It may be that the (latter) are better than the (former): Nor defame each other, nor call each other by (offensive) nicknames: Ill-seeming is a name connoting wickedness, (to be used of one) after he has believed: And those who do not desist are (indeed) doing wrong.” [Sûrah al-Hujurât: 11]
When we see the misfortune of another, we should supplicate on that person's behalf.
We should also thank Allah for showing favor to us, and we should say privately to Allah when we see someone else's misfortune: "Praise be to Allah who has spared me from what He afflicted you with, and who has blessed me with better than many of those whom He has created."
The Prophet (peace be upon him) tells us that anyone who says this supplication upon seeing the misfortune of another will be spared from that misfortune. [Sunan al-Tirmidhî (3431) an authenticated by al-Albânî in al-Silsilah al-Sahîhah (602)]