How a split-second Page-Not-Found flash revealed a hidden routing bug

A website that “mostly works” can be more dangerous than one that fails outright.

This is the story of a one-frame Page-Not-Found flash that appeared after clicking a splash screen — and how it led to uncovering a subtle but critical routing flaw: two valid entry points pointing to the same application.

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The Problem: A Flash That Shouldn’t Exist

Click splash →
blink “Page Not Found” →
homepage loads normally.

No console errors.
No PHP warnings.
No failed navigation.

But the flash was real — and repeatable.

This wasn’t a rendering glitch. It was a real HTTP request resolving to the wrong route for just long enough to be visible.

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Why This Bug Was Hard to See

Browser DevTools didn’t immediately reveal the problem:

• Network filters hid short-lived redirects

• Third-party iframes dominated the timeline

• The final destination looked correct

Only server-side logging at the front controller exposed the truth.

A single click triggered multiple document requests.

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The Root Cause: Two Entry Points, One Application

The application was accessible through two paths:

• /focus-local/index/...

• /focus-local/public/index/...

Only one was fully wired.

The splash screen linked to the wrong one first — then the app redirected itself to the correct path.

That first request briefly rendered a Page-Not-Found template before disappearing.

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Visualizing the Bug (Bad vs Good Flow)

❌ Broken Flow (Before)

flowchart-1          flowchart-2       flowchart-3       <====== Click a link.

Splash Click
   ↓
/focus-local/index/1     ← wrong entry point
   ↓
(Page Not Found renders briefly)
   ↓
Redirect
   ↓
/focus-local/public/index/1

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✅ Correct Flow (After)

flowchart-4           flowchart-5           flowchart-6       <====== Click a link

Splash Click
   ↓
/focus-local/public/index/1
   ↓
Homepage renders cleanly

One request.
One entry point.
No flash.

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The Fix: Canonical¹ URLs + Defensive Redirects

1. Fix the Splash Screen Link

Before

<a href="{{ doc_root }}index/1">

After

<a href="/focus-local/public/index/1">

No ambiguity. No guessing.

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2. Add a Safety Net in the Front Controller

In public/index.php, early — before routing:

if (strpos($_SERVER['REQUEST_URI'], '/focus-local/index/') === 0) {
    header(
        'Location: /focus-local/public' .
        substr($_SERVER['REQUEST_URI'], strlen('/focus-local')),
        true,
        302
    );
    exit;
}

Even if a bad link sneaks in later, the wrong page can never render.

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Why This Matters (Beyond This Bug)

This wasn’t a JavaScript issue.
It wasn’t a session issue.
It wasn’t a template issue.

It was an architectural hygiene issue.

If an application can be entered two ways, one of them will eventually be wrong.

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Debug Ghosts: Key Takeaways

• Log at the front controller when behavior feels impossible

• Don’t rely on DevTools alone for short-lived routing bugs

• Eliminate duplicate entry points early

• Treat “works most of the time” as a warning sign

Ghost bugs don’t disappear when ignored.
They disappear when you force the system to be unambiguous.

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TFOL Dev Series: Debug Ghosts

This post is part of the ongoing Debug Ghosts series — real-world debugging stories from rebuilding The Focus On Life platform.

📘 Part I

Debug Ghosts: How Twig and PHP Made Me Question Reality
→ Template inheritance, variable shadowing, and logic that looked right

📘 Part II

Debug Ghosts, Part II: Why Opening DevTools Fixed My Layout
→ CSS reflow, viewport scaling, and the illusion of “DevTools magic”

📘 Part III

Debug Ghosts, Part III: When Two Entry Points Become One

→ Canonical URLs, routing discipline, and killing the Page-Not-Found flash

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¹What “canonical” means (plain English)

Canonical means:

the single, authoritative, officially recognized version of something

In other words:
“This is the one true version — treat all others as copies.”

A canonical URL tells browsers and search engines:

“If this content appears at multiple URLs, this one is the real one.”

Example:

/focus-local/index/1 /focus-local/public/index/1 /focus-local/public/index/1?ref=splash 

These might all show the same page.

The canonical URL is the one you want everyone (and Google) to treat as the real page:

/focus-local/public/index/1 

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