Reverse Straw Man

A Straw Man argument is where your opponent takes you to be arguing for some B instead of A, proceeds to demolish B, and serves not-B up to you as a fait accompli. Reverse Straw Man is an extreme version of this where your opponent takes you to be arguing for not-A, proceeds to demolish not-A and slaps A in your face as complete and irrefutable. Even though you were arguing for A all along (or have pretended to, see below).

For obvious reasons it is difficult to bring off, and requires considerable skill at generating confusion about the initial purpose and objectives of the dispute, and so usually confined to domestic disputes, arguments with teenagers, drunken discussion in bars and so on. But it can be found in serious philosophical dispute, typically when your opponent realises with horror, halfway through, that they are wrong, and need to cover their tracks. Because of its inherently symmetrical nature, you can employ it yourself, i.e. pretend you were actually arguing for A along. When your opponent proudly presents you with irrefutable evidence for A, you reply that you gave that argument half an hour ago.

The tactic is best used in verbal discussion where it is difficult for anyone to remember what anyone said. If on an internet thread it is more difficult, as someone is bound to quote your very words back at you. A simple tactic is to accuse them of quoting you out of context, and perhaps throw in an accusation of bad faith: "It is quite clear I was arguing for A, please do not quote my words out of context". This has the risk of making them angry, but in that case they will either resort to capital letters and ranting, in which case they have lost, or they will present a carefully reasoned and detailed case that you could not have meant that, even in context. Because this requires supplying the original context, as well as a carefully reasoned and detailed argument around it, it will be too long for anyone to read or listen, and so they have also lost.

Straw man (which involves confusion about the aims and conclusion of the argument) is closely related to ground-shifting. Ground-shifting is when you change your argument or evidence half-way through the argument. There is a good characterisation of it here. The author correctly notes that the skillful employment of it requires deliberate and purposeful unclarity about definitions.