@Daan (the other one):

so for a layman: does that mean @Aljoscha's feed will be considered invalid? You made some posts with invalid strings in them, so any (new?) peer would now reject them, right?

No, fortunately not. The implementation that posted my (and @regular's) data substituted it with something that is safe to replicate.

The outcome of this discussion means that newer implementations do not need to perform that exact same data substitution, instead they can (and in certain circumstances must) completely reject invalid data, without trying to patch it up.