Strikethrough (aka line-through) text-styling would be a convenient way to identify errors, phrases that should be removed, etc.
You can use
In questions and answers, just write
<s>strikethrough</s>. You can also use
<strike>if you prefer.
In chat, the alternate syntax
---strikethrough---is used instead.
In comments, strikethrough is unsupported, but unicode tricks will work for some users in some browsers. If you really want strikethrough in comments, this is an option. Support will presumably improve over time.
For reasons unknown, Jeff Atwood has decided not to add an actual strikethrough button, so you'll have to use one of the methods above. See the feature request Add a strike-through to the editing interface on Meta Stack Exchange, currently marked "status-declined".