The most philosophically dangerous idea out there, to the best of my knowledge, is that the existence of "truth" can't be proven. If any one possible statement is true, then you can derive all of mathematics from a series of logical statements. Its one of those things that certain experts in the field know about but no one likes to talk about because discussing it doesn't get anyone anywhere.

People who just learn about this really, REALLY like to say stupid shit then when inevitably shown the stupidity of what they just said retreat to some variation of "all ideas are equally true" to avoid admitting they have no idea what the fuck they're talking about. If in this situation, tell them that the idea that you should kick them in the face is equally as valid as any idea to the contrary and therefore as a matter of philosophy it falls to individual choice.

and none of that has anything to do with Marxism which is a structural material economic and historical analysis