Inconsistent resolution.
WHY IT HAPPENS
A recent change, with different resolvers holding different cached answers.
Entirely normal during propagation.
HOW TO CONFIRM
Query your authoritative nameservers directly. If they return the correct answer, the change is made.
Then it is caching, and waiting is the answer.
A tool that queries from several locations shows how far the change has spread.
HOW LONG TO WAIT
Up to the previous TTL. If that was four hours, allow four hours.
Some resolvers cache longer than instructed, which is why the occasional person sees the old value well after everyone else.
IF IT PERSISTS BEYOND THE TTL
Check for conflicting records: two A records with different values, or a CNAME coexisting with other records.
Check whether nameservers from two providers are both listed at the registrar, which produces genuinely inconsistent answers.
WHAT TO TELL AFFECTED PEOPLE
To wait, or to try a different network.
Flushing their own DNS cache sometimes helps.
WHAT NOT TO DO
Change the record again. That restarts the clock and makes it worse.
