〜にかぎって

Meaning

There are two usages. One, as in 「うちの子にかぎって」, expresses the belief that this particular person is an exception and could never do wrong. The other, as in 「急いでいる時にかぎって」, laments the misfortune that inconvenient things happen precisely at such times.

Examples

あの真面目な彼にかぎって、嘘をつくはずがない Someone as earnest as he is could not possibly lie.
うちの子にかぎって、そんなことをするわけがない My child, of all people, would never do such a thing.
急いでいる時にかぎって、電車が遅れる It is always when I am in a hurry that the train is delayed.
約束した日にかぎって、雨が降る It always rains precisely on the day we made plans.