Why should the Erlenmeyer flask used to store $\ce{NaOH}$ be dry? Why should it be kept covered at all times?
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Without a proper closed container, sodium hydroxide readily attracts water and become a viscous solution (concentrated sodium hydroxide). This solution readily attracts carbon dioxide in air and reacts with it.
$$\ce{CO2(g) + H2O(l)<=>H2CO3(aq)}$$ $$\ce{2OH^{-}(aq) + H2CO3(aq) -> 2H2O(l) + CO3^{2-}(aq)}$$
(The $\ce{OH^{-}}$ in the second equation comes from $\ce{NaOH}$.)
This reaction decrease the amount of $\ce{NaOH}$ and thus the titration result will contains error.
Actually, the first equation can happen even without sodium hydroxide. Carbon dioxide in air is known to spontaneously dissolved into water and become carbonic acid. This is why the distilled/deionized water which in contact with air will have pH of around 6 after some time. With the presence of sodium hydroxide in water, the second equation greatly drives the equilibrium of the first solution to the right. Concentrated sodium hydroxide is used in some experiment as a carbon dioxide absorber.
Another possible problem that might occur is that concentrated sodium hydroxide can react with glass. Althought this is a very slow reaction, it might causes some unexpected error or ruin the glassware.
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$\begingroup$ The reaction with glass is not really a slow reaction, especially when it was a highly concentrated solution. $\endgroup$– Martin - マーチン ♦Commented Jun 8, 2015 at 12:28