The passage below comes from Emily's runaway imagination by Beverly Cleary.
During the week the stationmaster at the depot telephoned Mama to say that a crate of books had arrived from the state library. Mama had Grandpa pick up the crate in his Ford and deliver it to the Commercial Clubrooms. And wouldn’t you know?
That was the week the tomatoes were ripe and Mama was so busy canning tomatoes and selling tomatoes to other ladies who wanted to can them that she did not have time to go uptown and open that crate.
Could you explain to me the meaning of 'wouldn’t you know'? And if you don't mind, give me another example of this phrase with common words but with difficult nuance to me.