In a recent issue of Uncanny Avengers, a flashback to the 11th century AD shows a young Thor still using the axe Jarnbjorn, which he wielded prior to becoming worthy enough to lift Mjolnir. But earlier stories showed him already having Mjolnir before the time period in this issue, as far back as the mid 10th century and probably before then. So how come he's still got Jarnbjorn here? It doesn't make sense.
Or, honestly, it could just be a continuity error. With 70 years of publication history and counting, there are times when writers may not have access to all previous issues or information, and something they write contradicts something previously established.
Maybe he just wanted/needed a weapon with a cutting edge that day, something that could cleave instead of smash. Seems like it might have been an important part of why that enchantment worked as effectively as it did instead of how it might have worked on a blunt weapon like Mjolnir.
Spencerz, like the big one where Cain Marko aka Juggernaut went from being Charles Xavier's foster-brother (no blood relation; his father married Xavier's mother after Xavier's father died) to being his half-brother.
Or how the Genoshan Genegineer's son went from being a mutant molded into one of those mutate suits to being a non-mutant.
The first one is the one that bugs me the most; the mistake has been repeated so many times it seems like most of the current X-writers have bought into it.