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As an action, caltrops can be spread to cover a 5-foot-square area:

Any creature that enters the area must succeed on a DC 15 Dexterity saving throw or stop moving and take 1 piercing damage. Until the creature regains at least 1 hit point, its walking speed is reduced by 10 feet.

If a creature moves through several sets of caltrops, can they suffer this speed reduction multiple times?

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The speed reduction does not stack.

The rules for combining game effects state (DMG p. 252, added in the DMG errata; emphasis mine):

Different game features can affect a target at the same time. But when two or more game features have the same name, only the effects of one of them—the most potent one—apply while the durations of the effects overlap. For example, if a target is ignited by a fire elemental’s Fire Form trait, the ongoing fire damage doesn’t increase if the burning target is subjected to that trait again. Game features include spells, class features, feats, racial traits, monster abilities, and magic items.

So you cannot be affected by the speed reduction of more than one set of caltrops at a time.

But mundane items aren't on the list!

Indeed, "mundane items" is not on the list of examples given in the rule quoted above. This list is not meant to be exhaustive. To demonstrate this, we can observe that the official rules answers document, the Sage Advice Compendium, tells us that an opportunity attack is a game feature:

Can you use a melee spell attack to make an opportunity attack?

[...]

Each spell has a casting time. A game feature, such as an opportunity attack, doesn’t let you bypass that casting time, unless the feature says otherwise. [...]

Opportunity attacks, a type of reaction, are not "spells, class features, feats, racial traits, monster abilities, and magic items", so the list of game features given in the "same name" rule is not exhaustive.

However, the application of the rule needs to be exhaustive: excluding mundane items from this rule breaks the game. Alchemist's Fire is a mundane item:

This sticky, adhesive fluid ignites when exposed to air. As an action, you can throw this flask up to 20 feet, shattering it on impact. Make a ranged attack against a creature or object, treating the alchemist's fire as an improvised weapon. On a hit, the target takes 1d4 fire damage at the start of each of its turns. A creature can end this damage by using its action to make a DC 10 Dexterity check to extinguish the flames.

If we allow this feature to stack with itself, any creature can be quickly overwhelmed by stacking Alchemist's Fire effects. Let's walk through this:

  • Round 1: BBEG takes 4 hits from Alchemist's fire, suffers 4d4 fire damage, then uses its action to end one of the effects.
  • Round 2: 4 more hits, BBEG suffers 7d4 fire damage, then uses its action to end one of the effects.
  • Round 3: 4 more hits, BBEG suffers 10d4 fire damage, then uses its action to end one of the effects.

The damage just piles on, and the BBEG cannot keep up with the effects being added on.

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    \$\begingroup\$ @GuillaumeF. Yes, that is a list of examples. It makes no sense to say “magic item effects are game features, but mundane item effects are not game features.” The list of examples is not exhaustive - it would be exceedingly long if it included all of the things that were game features. \$\endgroup\$ Commented May 19, 2021 at 9:41
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    \$\begingroup\$ @GuillaumeF. No, it could not be construed that way, unless you approached it already motivated to exclude mundane items. If you’re the DM, you are free to rule however you like, but this is the rules as written answer. There are no hidden or secret rules. \$\endgroup\$ Commented May 19, 2021 at 10:12
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    \$\begingroup\$ @GuillaumeF. If this rule didn’t apply to mundane items, you could win any fight by stacking Alchemist Fire effects. It would break the game. \$\endgroup\$ Commented May 19, 2021 at 10:13
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    \$\begingroup\$ It also makes sense from a narrative perspective. You are either on fire or not on fire, it doesn't make sense that you exinguish the flames (ie to make yourself not on fire), but are somehow still on fire, particularly when the source of that fire is not a magic item or spell that is continually reapplying the fire. \$\endgroup\$
    – illustro
    Commented Jan 24, 2022 at 18:23
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    \$\begingroup\$ @Kirt It is spelled out in the rules, see my answer where I spell it out. \$\endgroup\$ Commented Jan 24, 2022 at 20:31

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