<p dir="auto">This surprised me; happens at both the REPL and in scripts (and no startup file).</p>
<div class="highlight highlight-source-julia notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="julia&gt; 1*2
2

julia&gt; a*b = b*a # errors as I expect:
ERROR: error in method definition: function Base.* must be explicitly imported to be extended
Stacktrace:
 [1] top-level scope
   @ none:0
 [2] top-level scope
   @ REPL[2]:1

julia&gt; 1=&gt;2
1 =&gt; 2

julia&gt; a=&gt;b = b=&gt;a
Error showing value of type UnionAll:
ERROR:
SYSTEM (REPL): showing an error caused an error
ERROR:
SYSTEM (REPL): caught exception of type StackOverflowError while trying to handle a nested exception; giving up

julia&gt; ┌ Error: Error in the keymap
│   exception =
│    StackOverflowError:
│    Stacktrace:
│     [1] Pair(a::Int64, b::Int64)
│       @ Main ./REPL[4]:1
└ @ REPL.LineEdit ~/Julia/master/usr/share/julia/stdlib/v1.7/REPL/src/LineEdit.jl:2536
julia&gt;
ERROR: fatal: error thrown and no exception handler available.
StackOverflowError()
Pair at ./REPL[4]:1
Pair at ./REPL[4]:1
# repeated many thousands of times... and crash

$ echo $?
1"><pre class="notranslate">julia<span class="pl-k">&gt;</span> <span class="pl-c1">1</span><span class="pl-k">*</span><span class="pl-c1">2</span>
<span class="pl-c1">2</span>

julia<span class="pl-k">&gt;</span> a<span class="pl-k">*</span>b <span class="pl-k">=</span> b<span class="pl-k">*</span>a <span class="pl-c"><span class="pl-c">#</span> errors as I expect:</span>
ERROR<span class="pl-k">:</span> error <span class="pl-k">in</span> method definition<span class="pl-k">:</span> <span class="pl-k">function</span> Base<span class="pl-k">.*</span> must be explicitly imported to be extended
Stacktrace<span class="pl-k">:</span>
 [<span class="pl-c1">1</span>] top<span class="pl-k">-</span>level scope
   @ none<span class="pl-k">:</span><span class="pl-c1">0</span>
 [<span class="pl-c1">2</span>] top<span class="pl-k">-</span>level scope
   @ REPL[<span class="pl-c1">2</span>]<span class="pl-k">:</span><span class="pl-c1">1</span>

julia<span class="pl-k">&gt;</span> <span class="pl-c1">1</span><span class="pl-k">=&gt;</span><span class="pl-c1">2</span>
<span class="pl-c1">1</span> <span class="pl-k">=&gt;</span> <span class="pl-c1">2</span>

julia<span class="pl-k">&gt;</span> a<span class="pl-k">=&gt;</span>b <span class="pl-k">=</span> b<span class="pl-k">=&gt;</span>a
Error showing value of type UnionAll<span class="pl-k">:</span>
ERROR<span class="pl-k">:</span>
SYSTEM (REPL)<span class="pl-k">:</span> showing an error caused an error
ERROR<span class="pl-k">:</span>
SYSTEM (REPL)<span class="pl-k">:</span> caught exception of type StackOverflowError <span class="pl-k">while</span> trying to handle a nested exception; giving up

julia<span class="pl-k">&gt;</span> ┌ Error<span class="pl-k">:</span> Error <span class="pl-k">in</span> the keymap
│   exception <span class="pl-k">=</span>
│    StackOverflowError<span class="pl-k">:</span>
│    Stacktrace<span class="pl-k">:</span>
│     [<span class="pl-c1">1</span>] <span class="pl-c1">Pair</span>(a<span class="pl-k">::</span><span class="pl-c1">Int64</span>, b<span class="pl-k">::</span><span class="pl-c1">Int64</span>)
│       @ Main <span class="pl-k">./</span>REPL[<span class="pl-c1">4</span>]<span class="pl-k">:</span><span class="pl-c1">1</span>
└ @ REPL<span class="pl-k">.</span>LineEdit <span class="pl-k">~</span><span class="pl-k">/</span>Julia<span class="pl-k">/</span>master<span class="pl-k">/</span>usr<span class="pl-k">/</span>share<span class="pl-k">/</span>julia<span class="pl-k">/</span>stdlib<span class="pl-k">/</span>v1.<span class="pl-c1">7</span><span class="pl-k">/</span>REPL<span class="pl-k">/</span>src<span class="pl-k">/</span>LineEdit<span class="pl-k">.</span>jl<span class="pl-k">:</span><span class="pl-c1">2536</span>
julia<span class="pl-k">&gt;</span>
ERROR<span class="pl-k">:</span> fatal<span class="pl-k">:</span> error thrown and no exception handler available.
<span class="pl-c1">StackOverflowError</span>()
Pair at <span class="pl-k">./</span>REPL[<span class="pl-c1">4</span>]<span class="pl-k">:</span><span class="pl-c1">1</span>
Pair at <span class="pl-k">./</span>REPL[<span class="pl-c1">4</span>]<span class="pl-k">:</span><span class="pl-c1">1</span>
<span class="pl-c"><span class="pl-c">#</span> repeated many thousands of times... and crash</span>

<span class="pl-k">$</span> echo <span class="pl-k">$</span>?
<span class="pl-c1">1</span></pre></div>
<p dir="auto">Not sure if this is intended:</p>
<div class="highlight highlight-source-julia notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="julia&gt; UUID
ERROR: UndefVarError: UUID not defined

julia&gt; using Random: UUID

julia&gt; UUID(x::String) = UUID(rand(UInt128)) # OK?
UUID

julia&gt; UUID(&quot;foo&quot;)
8c68d00a-33d1-777d-c4a1-0a41759d1581

julia&gt; using Base: getindex

julia&gt; struct Foo end

julia&gt; getindex(::Foo) = print(&quot;Hello&quot;)
ERROR: error in method definition: function Base.getindex must be explicitly imported to be extended"><pre class="notranslate">julia<span class="pl-k">&gt;</span> UUID
ERROR<span class="pl-k">:</span> UndefVarError<span class="pl-k">:</span> UUID not defined

julia<span class="pl-k">&gt;</span> <span class="pl-k">using</span> Random<span class="pl-k">:</span> UUID

julia<span class="pl-k">&gt;</span> <span class="pl-en">UUID</span>(x<span class="pl-k">::</span><span class="pl-c1">String</span>) <span class="pl-k">=</span> <span class="pl-c1">UUID</span>(<span class="pl-c1">rand</span>(UInt128)) <span class="pl-c"><span class="pl-c">#</span> OK?</span>
UUID

julia<span class="pl-k">&gt;</span> <span class="pl-c1">UUID</span>(<span class="pl-s"><span class="pl-pds">"</span>foo<span class="pl-pds">"</span></span>)
<span class="pl-c1">8</span>c68d00a<span class="pl-k">-</span><span class="pl-c1">33</span>d1<span class="pl-k">-</span><span class="pl-c1">777</span>d<span class="pl-k">-</span>c4a1<span class="pl-k">-</span><span class="pl-c1">0</span>a41759d1581

julia<span class="pl-k">&gt;</span> <span class="pl-k">using</span> Base<span class="pl-k">:</span> getindex

julia<span class="pl-k">&gt;</span> <span class="pl-k">struct</span> Foo <span class="pl-k">end</span>

julia<span class="pl-k">&gt;</span> <span class="pl-en">getindex</span>(<span class="pl-k">::</span><span class="pl-c1">Foo</span>) <span class="pl-k">=</span> <span class="pl-c1">print</span>(<span class="pl-s"><span class="pl-pds">"</span>Hello<span class="pl-pds">"</span></span>)
ERROR<span class="pl-k">:</span> error <span class="pl-k">in</span> method definition<span class="pl-k">:</span> <span class="pl-k">function</span> Base<span class="pl-k">.</span>getindex must be explicitly imported to be extended</pre></div>