This is more of a philosophical issue than anything, but it has jumped
up to bite us so I thought I'd make others aware;

Since NULL has no value they can be entered multiply times into unique
columns.

Some will say that NULL is a value and therefore should be unique in
this case (only one NULL allowed) and others will say that since NULL
has no intrinsic value it can be entered into a unique column as many
times as you would like. We have found this behavior in multiple
database types (MS-SQL, Oracle) so it is not unique to MySQL, it is just
where we noticed it.

It is not one of those things that we thought about....until now!