Cook spaghetti as directed, reserving some of the water used for cooking. Heat olive oil in large skillet if using pancetta. Otherwise cook bacon (without oil) until crisp. Toss the garlic into the fat and cook 1 minute to soften.
Add hot drained spaghetti to the pan and toss for 2 minutes to coat the strands. Beat the eggs and Parmesan together. Remove the pan from the heat and pour the egg/cheese mixture into the pasta, whisking quickly until the eggs thicken, but do not scramble. Thin out the sauce with a bit of the reserved pasta water until it reaches desired consistency. Season with pepper and salt.
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