diff --git a/base/abstractarray.jl b/base/abstractarray.jl
index af29aee6a74b6..2c5e3b2682642 100644
--- a/base/abstractarray.jl
+++ b/base/abstractarray.jl
@@ -2787,6 +2787,11 @@ colons go in this expression. The results are concatenated along the remaining d
 For example, if `dims` is `[1,2]` and `A` is 4-dimensional, `f` is called on `A[:,:,i,j]`
 for all `i` and `j`.
 
+Note that `mapslices` makes no guarantees about the order of execution. In addition, `f` is
+assumed to be [pure](https://en.wikipedia.org/wiki/Pure_function) when working with sparse
+data structures. Using an impure function with `mapslices` may cause bugs on sparse or 
+diagonal arrays, as `f` may only be called once on duplicated elements.
+
 See also [`eachcol`](@ref), [`eachslice`](@ref).
 
 # Examples
@@ -2940,6 +2945,11 @@ mapany(f, itr) = Any[f(x) for x in itr]
 Transform collection `c` by applying `f` to each element. For multiple collection arguments,
 apply `f` elementwise, and stop when when any of them is exhausted.
 
+Note that `map` makes no guarantees about the order of execution. In addition, `f` is assumed
+to be [pure](https://en.wikipedia.org/wiki/Pure_function) when working with sparse data
+structures. Using an impure function with `map` may cause bugs on sparse or diagonal arrays, 
+as `f` may only be called once on duplicated elements.
+
 See also [`map!`](@ref), [`foreach`](@ref), [`mapreduce`](@ref), [`mapslices`](@ref), [`zip`](@ref), [`Iterators.map`](@ref).
 
 # Examples
@@ -2997,6 +3007,11 @@ end
 Like [`map`](@ref), but stores the result in `destination` rather than a new
 collection. `destination` must be at least as large as the smallest collection.
 
+Note that `map!` makes no guarantees about the order of execution. In addition, `f` is assumed
+to be [pure](https://en.wikipedia.org/wiki/Pure_function) when working with sparse data
+structures. Using an impure function with `map!` may cause bugs on sparse or diagonal arrays, 
+as `f` may only be called once on duplicated elements.
+
 See also: [`map`](@ref), [`foreach`](@ref), [`zip`](@ref), [`copyto!`](@ref).
 
 # Examples
diff --git a/base/iterators.jl b/base/iterators.jl
index 1b96a24a9c16f..65452fbada9ab 100644
--- a/base/iterators.jl
+++ b/base/iterators.jl
@@ -27,8 +27,12 @@ export enumerate, zip, rest, countfrom, take, drop, takewhile, dropwhile, cycle,
 """
     Iterators.map(f, iterators...)
 
-Create a lazy mapping.  This is another syntax for writing
-`(f(args...) for args in zip(iterators...))`.
+Create a lazy mapping.
+
+When calling `Iterators.map`, `f` is assumed to be 
+[pure](https://en.wikipedia.org/wiki/Pure_function) if working with sparse 
+data structures. Using an impure function with `map` may cause bugs on sparse or diagonal
+arrays, as `f` may only be called once on duplicated elements.
 
 !!! compat "Julia 1.6"
     This function requires at least Julia 1.6.
diff --git a/base/reduce.jl b/base/reduce.jl
index 13e1b69c79ede..b4fa5eb851552 100644
--- a/base/reduce.jl
+++ b/base/reduce.jl
@@ -277,6 +277,11 @@ In general, it will be necessary to provide `init` to work with empty collection
 intermediate collection needs to be created. See documentation for [`reduce`](@ref) and
 [`map`](@ref).
 
+Note that `mapreduce` makes no guarantees about the order of execution. In addition, `f` is 
+assumed to be [pure](https://en.wikipedia.org/wiki/Pure_function) when working with sparse 
+data structures. Using an impure function with `mapreduce` may cause bugs on sparse or
+diagonal arrays, as `f` may only be called once on duplicated elements.
+
 !!! compat "Julia 1.2"
     `mapreduce` with multiple iterators requires Julia 1.2 or later.