Contents of [[Parsoidcrash]]:
<!-- Foo -->
Bar

Load page into the editor, change Bar to Baar, and save. You'll get an error. Wireshark reveals that the following happened:

POST /localhost/Parsoidcrash HTTP/1.0
Host: localhost:8000
Accept: */*
Accept-Encoding: deflate, gzip
Referer: http://localhost/wiki/API
User-Agent: MediaWiki/1.21alpha
Content-Length: 254
Content-Type: multipart/form-data; boundary=----------------------------5e7e7b667ce1

------------------------------5e7e7b667ce1
Content-Disposition: form-data; name="content"

<!-- Foo -->

<p data-ve-changed="{&quot;content&quot;:1}" data-parsoid="{&quot;dsr&quot;:[14,17,0,0]}">Baar</p>
------------------------------5e7e7b667ce1--
HTTP/1.1 500 Internal Server Error
X-Powered-By: Express
Content-Type: text/x-mediawiki; charset=UTF-8
Content-Length: 602
Connection: close

TypeError: Cannot call method 'cloneNode' of undefined

at [object Object].serializeDOM (/var/lib/parsoid/Parsoid/js/lib/mediawiki.WikitextSerializer.js:2330:16)
at /var/lib/parsoid/Parsoid/js/api/ParserService.js:593:55
at /var/lib/parsoid/Parsoid/js/api/ParserService.js:284:4
at /var/lib/parsoid/Parsoid/js/lib/mediawiki.parser.environment.js:185:3
at [object Object].<anonymous> (/var/lib/parsoid/Parsoid/js/lib/mediawiki.parser.environment.js:209:4)
at Array.1 (/var/lib/parsoid/Parsoid/js/lib/mediawiki.ApiRequest.js:60:4)
at EventEmitter._tickCallback (node.js:192:40)

Version: unspecified
Severity: critical