A similar security problem, though not as fundamentally baked into the format, is MIME parsing. The header section is supposed to be delimited from the body by an empty line (likewise for nested entities). But what if it's not? For better or worse, Sendmail was tolerant of the absence of an empty line and treated as headers everything up to the first line that didn't parse as a header or header continuation.[1] Later systems, like Postfix, originally copied this behavior. But Microsoft Exchange and Outlook are even more tolerant, yet in a much more horrendous way, by parsing as a header anything that looks like a Content-Type or related header immediately after the first empty line. They have similar hacks for other, similar violations. So today, depending on the receiving software, you can send messages that appear differently, including having different attachments. It's a security nightmare!
I not a Postel's Law hater, but ZIP archives and Microsoft's MIME parsing behaviors are just egregiously wrong and indefensible. And even if you think the Robustness Principle is inherently bad policy, you still have to design your formats, protocols, and systems to be as intrinsically constraining as possible. You can't rely on vendors adhering to a MUST rule in an RFC, unless it's unquestioningly crystal clear what the repercussions will be--everybody else will (because it's the natural and convenient thing to do) reject your output as trash and drop it on the floor immediately so violations never have a chance to get a foothold.
[1] MTAs don't necessarily need to care about MIME parsing, but Sendmail eventually gained features where parsing message contents mattered, setting the de facto norm (for those paying attention) until Microsoft came along.