And, yet, it does happen. The important part as the senior guy is to make the junior guy create a test case and then cut it to the bone until it is obvious where the bug is.
Story time: It's mid ninteen-ninety-mumble and your intrepid hero is a junior programmer handling multi-site integration and testing tool infrastructure. This being the time when the Swiss Army Chainsaw(tm) (aka Perl 4) is well and truly entrenched in the sysadmin and toolsmith programmers, my technical superiors throw me a couple of Perl books, set my deliverable date impossibly soon, and tell me to get going post haste.
So, I code. It's not a lot of code, but it is parsing and matching files in 3 different formats from 3 different sites. Of course, I hear what you are saying: "Perl and parsing is like mixing ammonia and chlorine--and probably more painful." Yes, I concur. But, it is the tool at hand in the long forgotten mists of time when RAM was expensive and spinning rust still resembled an iron brick. So, off I go with regexes for parsing (Yes, I know, now I have 3 problems).
And everything worked quite swimmingly. Except for a bit of idiocy that nobody could track down that occasionally flagged a couple of records as mismatched when manual inspection showed they really were not. Nobody minded that much as the scripts got 99.99% of the job done and didn't give a false negative, so: "Ship it, junior."
And so we did.
However, the bug annoyed me because I had to got clean up the false positives when they fired. And, if I am anything, I am a VERY lazy programmer--and this was preventing me from being lazy.
So, I eventually am waiting for one of the other groups to deliver, and I go spelunking for a testcase.
Spelunking? HAH! Cave diving is a better analogy.
The program took the most inclusive of the syntaxes, used each record from that to build a regex to look for the corresponding record in the other syntaxes, matched what it could and flagged what remained.
So, the program was building a dynamic regex on-the-fly and then using it. Not a huge deal, but the regexes were larger than most people were probably comfortable with. No problem, I validated this on much smaller records out to REALLY big records, and they work well.
Except for those weird cases ...
So, I'm looking through a case that works and trying to compare it to a case that doesn't. And I accidentally fat-finger some character set match and delete a character that shouldn't matter.
And the regex fails ... provoking the Asimovean "That's funny ..."
So I delete another character ... and it works again. What?!?!?!
So, I add a character. And it fails. And I add another. And it works again.
I stared at that regex for what felt like EONS until the light bulb went on.
The one that worked? 511 characters or 513 characters. The one that failed? 512 characters.
So, yes, I, a total Perl n00b managed to find a bug in Perl 4 in my first ever Perl program.
Sometimes the junior dude gets really unlucky and finds an actual system-level bug.