My own top contender for the worst writing advice is actually more about critique on the web:

Any advice in which a critic takes a piece of writing and rewrites it in their own style just to feed their own ego – prescribing how it "should" be done. Sadly, I've seen this multiple times here. No. Just no. 

Technical mistakes, yes – you can demonstrate the correct techniques ([b]if and only if[/b] you're sure they're mistakes) in terms of comma splices or typos or basic grammatical errors. However, if you're rewriting because in your mind it would make a piece flow better, or the word choice would be better off simpler/more complex, or because you don't like/understand semi-colons, you're bad at giving advice.

A lot of self-described writing experts on the internet think good writing begins and ends with a mastery of basic grammar, and that any piece can be improved with a line-by-line nitpick of possible typos and vague pseudo-errors like sentence fragments. Alternatively, perhaps they believe that show don't tell trumps all other elements of writing. Knowing what the rules are is a big part of the craft, yes. Knowing that it's okay to bend and even break them is what you learn whenever you take any class analyzing literature outside of high school. 

[@bluetommy] has it right when he says writing is rules-lite and it's all about style.

Some writers never play with the rules, because their voice doesn't require it. Some writers do it [i]all the damn time[/i], and that's their voice. Examples: Joseph Heller's run-on sentences in Catch-22; Vonnegut's conversational, fragmented style; Cormac McCarthy's, like, everything... That's not to say any random hobbyist on the internet is in the same ballpark, but there's no reason why their "mistakes" (often quirks, not errors) should be treated any differently just because they don't have the weight of a lifetime of publication behind them.

Nobody's taking bits of [i]these authors'[/i] writing and rewriting them in a different personal style, proclaiming with foolish confidence that it would be better their way.

To tie this in with my original complaint of rewriting masquerading as advice, there's no reason why anyone should say or imply that 'this would be better if you did it my way' when doling out specific advice on the internet. [i]Never[/i] stifle an author's voice. It's just unbearably rude, and 90% of the time the suggested rework is shit anyway, coming from internet randoms blinded by Dunning-Kruger.