I just noticed Headspace2 and wpSearch, two excellent plugins, don't play nice together. The problem is the Porter stemmer class both plugins use. Since it's the exact same class, the solution is easy: just get rid of the other (I went source diving and renamed the PorterStemmer class in Headspace2).
The problem wasn't obvious: what happened is that the post edit screen was cut off after the Headspace Meta Data fields, leaving the rest of the fields out. This had a side-effect of making comments closed in all entries I edited while under the influence of this problem.
It was quite a mess to figure out, so after I found the solution in my server error logs, I thought I'd post about it here so someone else can be spared from the trouble. These are two very good plugins and despite this conflict, I very much recommend using both of them.