I'm not a lawyer or an accountant, so this isn't legal or accounting advice. For that, you need a lawyer or an accountant. However . . .
It depends on whether you're living there or not. Just as if you were living in a single family home and needed the roof repaired, no--that wouldn't be deductible. That's a repair.
If it's an investment property and you're renting it out, then yes. It's an expense. Just as if you had a single family home that needed a roof repair. It's an expense and you can deduct it.
That's pretty much in line with Keith's point--if your other condo fees or HOA fees are deductible, then a special assessment would likely be, too. Otherwise, no.
I'm guessing that you yourself live in the condo and got hit with an unexpected assessment. (It happens. I've lived in condos.) In that case, the special assessment is just like the condo fee itself, making it (in all cases I'm aware of) not deductible.
But certainly check with an accountant to be sure.
Hope that helps.