Can I use an inner tube that's too narrow for the tyre? I recently got a puncture and when I went to replace the inner tube I discovered that I'd bought the wrong size tube. My tyre is 700 x 40c, the tube is 700 x 28-35c.

What is likely to happen if I use this tube? Will the tube explode when I attempt to inflate it?
In my experience smaller ones go in fine and last as per normal (not that I have pushed this to extremes with a 700x20 in a 700x40 but I only have 26x1.5 MTB tubes and they work fine in 26x2.00 tyres).

Allegedly putting a smaller tube in the tyre becomes a problem if and when you repair it, the patch doesn't necessarily stretch with the inner tube and it can come off.