This is due to a Parsoid bug.

Example that Moriel gave me: http://parsoid.wmflabs.org/he/%D7%93%D7%A0%D7%99%D7%90%D7%9C_%D7%A8%D7%93%D7%A7%D7%9C%D7%99%D7%A3

HTML starts with:

<html><head>...</head><body>
<!--{"@type":"mw:Transclusion","attrs":[{"name":"typeof","value":"mw:Transclusion"},{"name":"about","value":"#mwt1"},{"name":"id","value":"mwt1"},{"name":"data-mw-arginfo","value":"{\"dict\":{\"target\":{\"wt\":\"שחקנים \\n\",\"href\":\"./תבנית:שחקנים\"},\"params\":{\"שם\":{\"wt\":\"דניאל רדקליף\"},\"תמונה\":{\"wt\":\"[[קובץ:Daniel Radcliffe Paris 2012.jpg|200px]]\"},\"כיתוב\":{\"wt\":\"<small> דניאל רדקליף, [[2012]]</small>\"},\"תאריך לידה\":{\"wt\":\"23 ביולי 1989\"},\"מקום לידה\":{\"wt\":\"[[לונדון]], [[אנגליה]], [[הממלכה המאוחדת]]\"},\"כינוי\":{\"wt\":\"דן\"},\"דמות\":{\"wt\":\"[[הארי פוטר (דמות)|הארי פוטר]]\"},\"בכורה\":{\"wt\":\"בסדרה [[דייוויד קופרפילד]], 1999\"},\"אתר אינטרנט\":{\"wt\":\"http://www.danradcliffe.co.uk/\"},\"קישור\":{\"wt\":\"0705356\"}}},\"paramInfos\":[{\"k\":\"שם\",\"named\":true,\"spc\":[\"\",\"\",\"\",\" \\n\"]},{\"k\":\"תמונה\",\"named\":true,\"spc\":[\"\",\"\",\"\",\"\\n\"]},{\"k\":\"כיתוב\",\"named\":true,\"spc\":[\"\",\"\",\"\",\"\\n\"]},{\"k\":\"תאריך לידה\",\"named\":true,\"spc\":[\"\",\"\",\"\",\"\\n\"]},{\"k\":\"מקום לידה\",\"named\":true,\"spc\":[\"\",\"\",\"\",\"\\n\"]},{\"k\":\"כינוי\",\"named\":true,\"spc\":[\"\",\"\",\"\",\"\\n\"]},{\"k\":\"דמות\",\"named\":true,\"spc\":[\"\",\"\",\"\",\"\\n\"]},{\"k\":\"בכורה\",\"named\":true,\"spc\":[\"\",\"\",\"\",\"\\n\"]},{\"k\":\"אתר אינטרנט\",\"named\":true,\"spc\":[\"\",\"\",\"\",\"\\n\"]},{\"k\":\"קישור\",\"named\":true,\"spc\":[\"\",\"\",\"\",\"\\n\"]}]}"},{"name":"data-parsoid","value":"{\"tsr\":[0,356],\"src\":\"{{שחקנים \\n|שם=דניאל רדקליף \\n|תמונה=[[קובץ:Daniel Radcliffe Paris 2012.jpg|200px]]\\n|כיתוב=<small> דניאל רדקליף, [[2012]]</small>\\n|תאריך לידה=23 ביולי 1989\\n|מקום לידה=[[לונדון]], [[אנגליה]], [[הממלכה המאוחדת]]\\n|כינוי=דן\\n|דמות=[[הארי פוטר (דמות)|הארי פוטר]]\\n|בכורה=בסדרה [[דייוויד קופרפילד]], 1999\\n|אתר אינטרנט=http://www.danradcliffe.co.uk/\\n|קישור=0705356\\n}}\",\"tagId\":1}"}]}-->
<table class="infobox" style="width:18em; font-size:85%;" cellspacing="5" about="#mwt1" data-parsoid="{}">[....]</table>
<meta typeof="mw:Transclusion/End" about="#mwt1" data-parsoid="{&quot;dsr&quot;:[null,356,null,null]}">

Note how:
* The transclusion starts with a comment (!) which tries to be in the #mwt1 about group
* The transclusion continues with a table in that same about group
* The about group ends with a meta tag in that same about group

This doesn't serialize cleanly at all, the template gets inserted into the wikitext and the table gets serialized as wikitext as well. Even the <meta> tag gets serialized: http://parsoid.wmflabs.org/_rt/he/%D7%93%D7%A0%D7%99%D7%90%D7%9C_%D7%A8%D7%93%D7%A7%D7%9C%D7%99%D7%A3

In VisualEditor, this causes:
* corruption on save like in the diff Moriel linked
* the user to be able to edit inside the infobox, because the comment is ignored and there are no attributes on the <table> telling VE it's part of a template (there's about="#mwt1" but that doesn't refer back to a valid about group, so it's interpreted as the start of an about group)