Actually, user B might have a problem as well after adding/editing a Statement's reference.
After that change, all other Statement's references can not be edited and no new ones can be added for those.

Tried the following example:

• I have an item with 2 claims: claim A and claim B which both have a reference
• I change the reference of claim A
• I try to change the reference of claim B (without reloading the page): this fails

What happens:

• the request fails, because EditEntity::fixEditConflict() returns false in line 481.
• of course the problem lies deeper, so I tracked it down a bit more
• in line 468 of EditEntity::fixEditConflict() $patchedCurrent seems to contain wrong data: Claim B still has the old reference although it should have been patched with the Diff between the baseContent and the newContent (which still is correct - line 458).
• consequently this leads to the case where the currentContent and the patched currentContent are the same and diffing them results in an empty diff which then leads to the patch collapsing.
• going even further, in line 855 of Entity::patch() both parameters (array of claims of current content & the claimsDiff) seem to be correct, so I assume the problem might be somewhere deeper in the MapPatcher of the Diff extension

It runs into line 130 in MapPatcher::getPatchedMap(): "Cannot do a non-add operation with an element not present in a map."

related changesets:
for wikibase: https://gerrit.wikimedia.org/r/#/c/60872/
for diff: https://gerrit.wikimedia.org/r/#/c/60870/