Actually I'm not sure if that's really a bug, though it does look like one to me.

In the mongo shell:

> ISODate("19990000")

returns ISODate("1998-11-30T00:00:00Z")

That is, 1999-01-01 would be 1 January 1999, then 1999-00-00 is 1 January 1999 minus one month minus one day, so 30 November 1998

> ISODate("12345678")

returns ISODate("1238-10-17T00:00:00Z")

Because by the same reasoning, 1234-56-78 is 1234-01-01 plus 55 months and 77 days, which is 17 October 1238.

The #Wikipedia page on ISO 8601 states that months and days must be between 01 and 12 and between 01 and 31 respectively. I couldn't find a source that states otherwise. If that's true, then "19990000" and "12345678" are both invalid date strings; as such I would expect the ISODate() function to throw an error just like it does for other invalid inputs.