As for those who pass by their site of ihram without assuming it, they have to return to that site and assume ihram, for it is a manageable obligation that should never be neglected. So, if one does not go back and assume ihram from the right site – as some mistakenly do from Jeddah and the like – one in this case will be obliged to slaughter a sacrificial animal in expiation, i.e. to slaughter a sheep or to get one-seventh of a camel or one-seventh of a cow and divide it among the residents of the Sanctuary of Mecca, provided one (the pilgrim who sacrifices) should eat nothing from it.
In a nutshell, Muslims should take an interest in their religious matters so as to perform all acts of worship properly and perfectly as legally enjoined.
Among these religious matters is the ihram for Hajj and ‘Umrah; it has to be assumed from the sites for ihram specified by the Prophet (PBUH). Therefore, a Muslim should be aware of these sites and never pass them without being in a state of ihram.