Do your nameservers say the same thing?
Your domain has one answer, served by several machines that are meant to keep identical copies. We ask each of them the same questions directly, with no resolver in between, and show you where they differ.
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Questions
How can nameservers for one domain give different answers?
A domain's records live on several servers that copy from each other. If a copy stops updating, that server keeps handing out the old records. Resolvers pick a nameserver more or less at random, so some visitors get the new answer and some the old, and it comes and goes depending on who asks.
Why doesn't a normal DNS lookup show this?
A normal lookup goes through a resolver, which asks one nameserver and caches the reply. You see one answer and never learn the others differ. This check skips the resolver and asks each nameserver directly with recursion turned off.
What is a lame delegation?
The registry says a server handles your domain, but that server doesn't: it refuses, or it answers without the authoritative flag. Resolvers that pick it get an error or a delay before trying the next one.
What is glue, and how does it drift?
When a nameserver's name sits inside the domain it serves, like ns1.example.com for example.com, the registry stores its IP address too, so resolvers can find it. That copy is the glue. If the server moves and only the zone gets updated, the registry keeps sending people to the old address.
My A records differ between nameservers. Is that a problem?
Usually not. Geo-routing and load balancing hand out different addresses on purpose, so we report that as a note. Differences in MX, TXT or the NS set on the same serial have no such excuse, and we report them as faults.
Can this miss something?
Yes. Most large nameservers are anycast, one address served from many places, and we only reach the instance nearest our server. A fault in a distant instance won't show. We also only reach IPv4 addresses, and a change made in the last few minutes can look like a disagreement until it finishes copying.
For one zone's serial history and expire clock, see the SOA checker. To see what resolvers around the world have cached, see propagation.