Sheikh Hânî al-Jubayr, judge at the Jeddah Supreme Court
In case of forgetfulness you do not have to expiate for your oath.
Allah says: “Our Lord, condemn us not if we forget or fall into error” [Sûrah al-Baqarah: 286]
The Prophet (peace be upon him) said: “My nation will not be held accounted for what they forgot or were coerced into doing” [Sunan Ibn Mâjah, Sunan al-Dârqutnî and others. It was authenticated by al-Dhahabî]
And Allah knows best.
Source: Islam Today