How could condo board not warn us of assessment for repairs?

I moved into my condo a year ago and was elected to the condo board a month ago. The seller of my condo paid approximately $14K in special assessments at closing for two separate repair projects. Now the condo board is advising the other board members that a $600K "cost overrun" special assessment must be levied against all association members. Is this common and/or legal? Shouldn't we expect the board to have known about these additional costs when the work was being done? Note, both projects have been finished for at least two years now.