catch {
exec('rm -rf /*')
}
Back in my days working as .NET developer on Windows 7, I came into work one morning to find a colleague fuming that his machine had died on him.
He spent the whole morning reinstalling Windows and getting his environment set back up, and then pulled the branch he was working on, happy to finally be done with setup and get back to work. Ran his test suite and bam, machine crashes!
It was only at that point the penny dropped. We took a look at his branch, and sure enough he'd accidentally written a test that, when ran, deleted his entire C: drive!
That particular lesson made me very careful when writing any code that does things with the filesystem.