Opinions vary on how good an idea the robustness principle is. That is why, for example, the XML standard requires a conforming validator to throw an error on invalid XML.

In our modern world, the robustness principle has become an invitation to security bugs, and vendor lock-in. Edge cases snuck through one system on robustness, then trigger unfortunate behavior when they hit a different system. Two systems tried to do something reasonable on an ambiguous case, but did it differently, leading to software that works on one, failing to work on the other.