I had an associative array with class objects as the values, and I'd use the "in" operator in several places to see if there were particular keys in the array, then access aarr[foo] to access the class object and pass it to a function or something. In a couple places, I forgot to check with "in", if it existed, and it ended up adding a null ref to the array, and so it would crash when accessing members. It took me a little while to find out how null references got into the array, but I was thinking that it could throw an exception if the key wasn't in the array. It would force you to test if it was in there using "in" if you weren't sure, and explicitly add a value if you wanted one in there.

Object[int] aarr;
aarr[3] = new Object; /* as usual */
if(4 in aarr) something(aarr[4]);  /*  safe  */
something(aarr[5]);  /* oops */

Sure it was a pretty stupid mistake of mine, but this could be considered a type of "bounds checking" ;)