It seems like the user in question is using the "combining long stroke overlay" combining character to create the strikethrough effect. I think using this combining character causes the entire struck-through text to be treated as a single long word, which in turns stretches the table cell/row. Hence the text overflow in some browsers. Somehow Chrome has a way to break this text into multiple rows though.
You can see the combining character separately if you paste the strike-through text (reproduced below for your convenience) in an address bar.
2̶.̶)̶ ̶W̶h̶y̶ ̶d̶o̶ ̶p̶e̶o̶p̶l̶e̶ ̶s̶a̶y̶ ̶i̶t̶'̶s̶ ̶b̶a̶d̶ ̶p̶r̶a̶c̶t̶i̶c̶e̶?̶ ̶I̶ ̶a̶c̶t̶u̶a̶l̶l̶y̶ ̶f̶i̶n̶d̶ ̶i̶t̶ ̶m̶u̶c̶h̶ ̶e̶a̶s̶i̶e̶r̶ ̶t̶o̶ ̶r̶e̶l̶i̶a̶b̶l̶y̶ ̶r̶e̶t̶u̶r̶n̶ ̶4̶0̶4̶s̶ ̶w̶i̶t̶h̶ ̶t̶h̶i̶s̶ ̶m̶e̶t̶h̶o̶d̶ ̶t̶h̶a̶n̶ ̶a̶n̶y̶t̶h̶i̶n̶g̶ ̶e̶l̶s̶e̶ ̶I̶'̶v̶e̶ ̶t̶r̶i̶e̶d̶ ̶b̶e̶f̶o̶r̶e̶.̶.̶.̶
If the strikethrough effect were created via HTML elements (<s>, <del>) or CSS text-decoration property, then the layout wouldn't break (I tested by adding <s> tag around some comment text in Firebug).
I'm not well-versed in web technologies to suggest what can be done about this. I think strikethrough formatting is not supported in SE comment in the first place because like James said
Comments should be short/simple for what one wants to say, not go into what one didn't want to say
So adding proper strikethrough formatting to comments might not be the right approach. I guess we can filter out that specific combining character instead? Or perhaps it's a rendering bug that depends on browser vendors to iron out?
