As a DM I have infinite dragons. Or dreadnought fire giants which almost led to a TPK in my last session, something that is not all that unusual.The game is written in a way that nothing is supposed to actually challenge the characters.

As a DM I have infinite dragons. Or dreadnought fire giants which almost led to a TPK in my last session, something that is not all that unusual.The game is written in a way that nothing is supposed to actually challenge the characters.
The game is written in a way that nothing is supposed to actually challenge the characters.
Sure, but let's say that the spell cast was actually Misdirected Rope Trick Blender. This spell hides the caster and makes an illusory version of them climb a rope trick that leads into a magical blender of force (like a Blade Barrier), which activates one round after a creature enters the space.the Orcs know magic happens and can see that their opponent has just climbed a rope - theres nothing to stop them climbing the same rope and assuming their target will be there at the top
There's nothing stopping them from assuming that, but there's also nothing stopping them from assuming it's a spell that protects the caster and harms their opponents. For all they know it teleports the caster away and leaves a gate to hell at the top of the rope. So again, if the DM uses their own knowledge of how the spell works to determine monster actions, without considering what the monsters would know about it, that's a problem.the Orcs know magic happens and can see that their opponent has just climbed a rope - theres nothing to stop them climbing the same rope and assuming their target will be there at the top
No intent was assumed (except by you, in this post). People behave inappropriately all the time without intending to.Sure, but your response assumed negative intent and was snarky. One could say that you should or could "do better."
Which is great, until you realize that people can think they understand how a spell works (and confidently so), but be wrong about it. Happens all the time.I disagree. As a DM I don't know every spell in the PHB so if a player casts a spell that I'm not 100% sure of I'll quickly look it up if their description of how it works or how they think it works seems off, or contradictory how I think it works. In the end I think the player should know how a spell works, or if they are confused or its ambiguous, ask out of game for clarification before they cast it in game or suffer the consequences. You'd think as a player who is running a spellcaster they'd read their spell descriptions before getting to the table to familiarize themselves beforehand not cry foul after the fact.
Sure, but let's say that the spell cast was actually Misdirected Rope Trick Blender. This spell hides the caster and makes an illusory version of them climb a rope trick that leads into a magical blender of force (like a Blade Barrier), which activates one round after a creature enters the space.
I think it's quite relevant to ask whether ALL of those enemies would have climbed the "rope trick" had the rope trick actually been this other spell. If they would have, no foul (though the talk of a "woodshed" does concern me a bit). Otherwise, most definitely a foul on the DM's part, IMO, since they were clearly metagaming in order to punish the player.
The difference is that it's magic, and you can't know the effect of a spell just from outward appearances. For all they know there's some kind of magical protection at the top of the rope that zaps evil creatures.I don't see any difference just because it's a spell.
This is exactly what I would have done as DM. Point it out to the player, because it's sure as naughty word something the ranger in-world would understand. If they still want to do it, that's up to them.However, I would warn the player that this would happen, that there's nothing stopping the enemy from following.