Repeating date fields cannot be reused as non-repeating date fields in another content type

This may be a problem with CCK and not with Date specifically, but I've only seen it in date fields...

When you add a field to a content type, you select the widget you want to use. There is nothing to prevent you from selecting a repeating date widget on one content type and a non-repeating date widget on another content type, with the same field. However, if you do this, one or the other will disappear from the node/add form. Editing the field settings and saving them (without changes) cause the field to resume working on the one content type and stop working on the other.

It's not just the widget that disappears -- an entire column of the database table comes and goes with these changes; however, because of the way repeating dates are stored, actual data do not appear to be lost when this happens.