@tshenry In case it might help with debugging, this has happened twice on our discourse.group forum as well. Here’s a link to a mysteriously closed topic (already reopened): https://igraph.discourse.group/t/how-should-i-do-for-reducing-graph-object-memory-usage/909
I asked about this on Discourse Meta here:
EDIT: It seems that this happens when the user who posted attempts to delete their post. Deleted posts appear locked to outside viewers, without an indication that the post was in fact deleted (and not just locked). The good news is that the person who posted can “unlock” it by undeleting. See the above-linked topic for more details.