We recently assisted with a migration of DNS services to the Network Solutions (worldnic.com) which is a pretty normal operation.
The process went as normal, transferred the domain to Network Solutions (“NS”), waited a few days for that to process.
Once in Network Solutions the WHOIS information was updated and again we waited 48 hours for the next move.
A zone-dump was acquired from the existing DNS provider, including all A, CNAME, SRV and TXT records.
We initiated the DNS migration (Switch NS) process at NS and then proceeded to update their DNS with the proper records.
After two hours the settings were verified via direct query using dig.
All was well.
About eight hours later the client called, they had noticed an issue on the web-site.
Imagine our surprise when, upon examining the DNS records we found two new records that we did not create!
Apparently, some time between 11pm and 7am these two records were added.
The two A records were for @ (i.e.: domain.tld) and www.
They pointed to a parking page IPv4 address run by NS.
Our existing records were still in-place.
The client was seeing the issue due to the round-robin effect of these two addresses.
Some times seeing the proper site/host and other times not.
Simply deleting these spurious records and waiting 7200 seconds did the trick.