The latter is more commonly encountered than the former even though it is usually the former that is meant.

forego means to go before or precede

You are unlikely to come across this usage very often, except in the phrase a foregone conclusion. You cannot, for example, forego your breakfast, walk in the park or pre-dinner cocktail.

forgo means to to do without

If the weather is poor, we will have to forgo our ascent of Ben Vorlich.