Exercise 3: The Undefined WC_Cart Fatal Error — Possible Solution
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THE LIKELY CAUSE
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Per this chapter, this is the WooCommerce-specific timing gotcha
named directly: "code that references WooCommerce's own classes or
functions must not run before WooCommerce itself has finished
loading - a fatal error results if a class like WC_Cart is referenced
before it exists yet." The plugin's code is very likely running too
early - before WooCommerce's own plugin has finished loading and
defined its classes - which can happen depending on plugin load
order.

THE TWO FIXES THIS CHAPTER NAMES
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Per this chapter, the two fixes are:
1. Wrapping the WooCommerce-dependent code inside a plugins_loaded
   hook, so it only runs after all active plugins (including
   WooCommerce) have finished loading.
2. Checking class_exists('WooCommerce') first, and only running the
   dependent code if that check passes.

WHY THIS PROBLEM CAN APPEAR INTERMITTENTLY
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Per this chapter, this failure mode "otherwise tends to appear
intermittently depending on plugin load order" - meaning the exact
same code might work or fail depending on the order plugins happen to
load in, which is part of why it's worth guarding against explicitly
rather than assuming a particular load order will always hold.

WHY THIS WORKS AS AN ANSWER
------------------------------
It correctly identifies premature execution before WooCommerce has
loaded as the cause, and correctly names both fixes this chapter
specifically recommends, along with why the bug can appear
inconsistently.