Using a short handle on a Merkur 25C, using to much pressure, and trying to squeeze out a fourth shave from a Personna blue lab yields irritation.
When you're using a new razor, one that you've been warned is more aggressive (Executive Shaving Braveheart), and you're in a hurry, don't treat it like a Gillette Slim Twist, and don't use a pre-shave oil that you forgot sometimes causes mild irritation, and don't go against the grain toward the end of the second pass, and don't let your lather get a bit too dry and think "Och, never mind" and don't apply rose water before alum, and don't go over your chin, where - until recently - you had a beard - It doesn't end well, you dolt. (No weepers, no cuts, but I wouldn't look out of place at the local burns unit in the hospital)