While we do mention it as a limitation of behaviours, the fact that you cannot reliably have multiple behaviours on a single field is not prominently documented or flagged up in the UI. This, in combination with a lack of validation that a user has not tried to put more than one behaviour on a field, can lead to customers inadvertently using behaviours in the wrong way without any obvious indication that they have done so.
The documentation could also possibly stand to be clearer on what counts as 'a single field': at the moment it wouldn't be unreasonable to imagine that having multiple behaviours was OK as long as the mappings were different, or that a more 'specific' mapping (e.g. that specified a particular project and issue type) would supersede a more 'general' one (e.g. that applied to all projects and issue types) wherever both mappings applied.