diff --git a/NEWS.md b/NEWS.md
index 9cc478b29cb48..bac729c4e7a5e 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -126,6 +126,9 @@ Standard library changes
 
 * `mmap` is now exported ([#39816]).
 
+#### DelimitedFiles
+
+* `readdlm` now defaults to `use_mmap=false` on all OSes for consistent reliability in abnormal filesystem situations ([#40415]).
 
 Deprecated or removed
 ---------------------
diff --git a/stdlib/DelimitedFiles/src/DelimitedFiles.jl b/stdlib/DelimitedFiles/src/DelimitedFiles.jl
index bf88c12ea9cf2..3091fed79fee8 100644
--- a/stdlib/DelimitedFiles/src/DelimitedFiles.jl
+++ b/stdlib/DelimitedFiles/src/DelimitedFiles.jl
@@ -190,8 +190,9 @@ Specifying `skipstart` will ignore the corresponding number of initial lines fro
 If `skipblanks` is `true`, blank lines in the input will be ignored.
 
 If `use_mmap` is `true`, the file specified by `source` is memory mapped for potential
-speedups. Default is `true` except on Windows. On Windows, you may want to specify `true` if
-the file is large, and is only read once and not written to.
+speedups if the file is large. Default is `false'. On a Windows filesystem, `use_mmap` should not be set
+to `true` unless the file is only read once and is also not written to.
+Some edge cases exist where an OS is Unix-like but the filesystem is Windows-like.
 
 If `quotes` is `true`, columns enclosed within double-quote (\") characters are allowed to
 contain new lines and column delimiters. Double-quote characters within a quoted field must
@@ -232,7 +233,7 @@ readdlm_auto(input::IO, dlm::AbstractChar, T::Type, eol::AbstractChar, auto::Boo
 function readdlm_auto(input::AbstractString, dlm::AbstractChar, T::Type, eol::AbstractChar, auto::Bool; opts...)
     isfile(input) || throw(ArgumentError("Cannot open \'$input\': not a file"))
     optsd = val_opts(opts)
-    use_mmap = get(optsd, :use_mmap, Sys.iswindows() ? false : true)
+    use_mmap = get(optsd, :use_mmap, false)
     fsz = filesize(input)
     if use_mmap && fsz > 0 && fsz < typemax(Int)
         a = open(input, "r") do f