Had that happen at work. I just drag-and-dropped a file into the Outlook web-UI, thinking it’d attach as an e-mail. Turns out, they recently changed that feature and you now have to drop into the right half of the area. If you drop into the left half, it uploads into OneDrive.
I accidentally did that. The document had personal data inside. That’s a breach of GDPR. Fucking ace.
(I’m not sure that attaching to the e-mail isn’t also a breach of the GDPR, since my company switched Microsoft 365 for various things. But yeah, I certainly would have liked a confirmation dialog.)