There's a low-rate error logs in RESTBase logs related to some crash in Parsoid code. Log example:
Request URI: /ca.wikipedia.org/v1/transform/html/to/wikitext/Pisa_daurada
Error stack:
Cannot read property 'caption' of undefined TypeError: Cannot read property 'caption' of undefined at Object.<anonymous> (/srv/deployment/parsoid/deploy-cache/revs/465f9c47ebefadc71a5f38b19e57e3ea820d3bc3/src/lib/ext/Gallery/index.js:308:24)
Unfortunately we don't know what content was provided in the transform POST request.