One noob came to me with a serious codegen bug in GCC, where even with `-O0` it would fail to correctly run a trivial for loop. Another found a huge security hole in `sudo` that gave everyone unrestricted access. My favorite was one who asked if the JDK standard library had any known bugs processing the letter "g".
They all turned out to be user error, if you can believe it.
But I also have a good counter-example story:
Some day I found an HTTP rfc violation in one of very popular oss HTTP client libraries. I filed a bug report with detailed reproduction. It was closed immediately, with "works as designed, you misunderstood HTTP". Then we had a debate in ticket comments for many days and I couldn't convince them to the right interpretation of HTTP (I admit, the text is not easy sometimes). Finally I posted a message on HTTP mailing list and Roy Fielding confirmed I was right. They reopened and fixed.
I must say this is really hard thing to argue with somebody with an edge in experience and not come out as arrogant.
In particular - when somebody responds with "I have more experience / I've been doing it for 20 years, and you say I'm wrong?". How to best handle such cases?
I wish I knew. I try to limit the discussion to the purely technical, or to barely acknowledge it as in "sure, but RFC123 says X and Y implements it that way as shown in Z".
Of course, that goes for when I'm the authority as well. I don't care who is correct, I care about what.
Even though I'm still juniorish, I still run into issues like this that stump me. But then occasionally you do find bugs with existing software which keeps you second guessing everything. Usually those bugs come from using two things in conjunction that haven't been well tested together.
Also sometimes, you find a bug that isn't accepted by the software vendor/owning team as a bug, because it has some sort of obscure work around that would take you a week of tinkering to figure out. Those "aren't bugs" but yeah, they are bugs. Software vendors that also sell consulting and related services love to pull shit like that