Properties may have [[https://www.wikidata.org/wiki/Help:Property_constraints_portal|property constraints]] that are defined by the community. If a user submits a value that violates a constraint, we should still save it, but we need to inform users that the value may break the rules.
On the technical side, we need to make some kind of API call during the submission step that will test against a given property's constraints.
On the design side, we need to figure out how to best show the resulting message to the user at both the "item level" (rows below the input field in the statement panel) as well as at the qualifier level. We should follow the wikidata paradigm of displaying constraint violations as warnings and not errors, and we likely will allow the data to be saved. Ideally, in the warning message, we would link to the constraints portion of the relevant wikidata item so the user can understand where the constraint is coming from.
We should support the display of constraint violation messages for any property that has constraints, regardless of data type.