diff --git a/pack-dist.ps1 b/pack-dist.ps1
index 15210dd..cbac0fe 100644
--- a/pack-dist.ps1
+++ b/pack-dist.ps1
@@ -27,6 +27,13 @@ if (-not (Test-Path $exe)) {
     throw "Release\rpl4opt.exe not found - build first (see BUILD.md 2)."
 }
 
+# Refuse to touch a dist the game is currently running from.
+$running = Get-Process rpl4opt -ErrorAction SilentlyContinue |
+    Where-Object { $_.Path -like "$dist\*" }
+if ($running) {
+    throw "rpl4opt.exe is running from $dist (PID $($running.Id)) - close the game first."
+}
+
 Write-Host "Packing into $dist"
 if (Test-Path $dist) { Remove-Item -Recurse -Force $dist }
 New-Item -ItemType Directory -Force "$dist\SPOOLS" | Out-Null