I discovered this when trying to merge nodes using apoc.refactor.mergeNodes procedure.
If a node I'm merging has a relationship of type with a unique constraint, then the merger operation fails with IndexEntryConflictException.

Neo4j Version: 5.25.1 + APOC 5.25.1
Operating System: macOS 15.1.1
API: Docker

Steps to reproduce

1. Start with an empty DB.
2. Run the following to create the constraint and populate the DB with some nodes and relationships:

CREATE CONSTRAINT foo_uniq FOR ()-[r:MY_REL]-() REQUIRE r.foo IS UNIQUE;
CREATE
    (n1 {name: "n1"})-[r1:MY_REL {foo: "a"}]->(n2 {name: "n2"}),
    (n3 {name: "n3"})-[r2:MY_REL {foo: "b"}]->(n4 {name: "n4"});

3. Attempt to merge nodes n1 and n3:

MATCH (n1 {name: "n1"}), (n3 {name: "n3"})
CALL apoc.refactor.mergeNodes([n1, n3]) YIELD node
FINISH

Expected behavior

Nodes are merged and we're left with:
(n1)-[r1]->(n2)
(n1)-[r2]->(n4)

This is what happens if foo_uniq constraint is not present.

Actual behavior

The query returns:

Failed to invoke procedure apoc.refactor.mergeNodes: Caused by: IndexEntryConflictException{propertyValues=( String("b") ), addedEntityId=-1, existingEntityId=1}