UPDATE: Point 4 was changed at the author's request to clarify his meaning after receiving many questions. This version was updates as of May 3, 2017.
Pathfinder has a lot of rules. We might think we know those rules pretty well, but it often pays to crack the book to actually look at them from time to time. While you might remember how to calculate your to-hit bonus, or that you get bonus spells based on a high casting stat, there are a lot of other rules you might remember incorrectly, and to your detriment.
Rule #1: The Heal Skill Can, In Fact, Restore Hit Points
Most of us don't bother investing points into the Heal skill. Sure you can use it to stop a party member from bleeding out, or to figure out what sort of wound killed a man you find in a dungeon, but what else can you do with the skill?
Quite a lot, as it turns out.
According to page 98 and 99 of the Core Rulebook, you can make Heal checks to treat deadly wounds. If you have a healer's kit, and expend 2 of the uses in it, you can make a check against a DC 20. Success means the character heals a number of points equal to their level. If you beat the DC by 5, they also heal a number of hit points equal to your Wisdom modifier. You can only do this for wounds acquired in the past 24 hours, and never more than once per day.
This is in addition to treating disease, poison, and long-term care. So, in the future, it might be worth investing a few points.
Rule #2: The Difference Between Being Flat-Footed, and The Surprise Round
Being ambushed is something that happens with a fair bit of frequency in Pathfinder, but when combat starts and only some people are aware of it, you get a surprise round according to page 178 of the Core Rulebook. Everyone who is aware combat is happening (the ambushers, and sometimes everyone in the other group who makes a high enough Perception check) gets to act in the surprise round. You get a single standard or move action, as well as free actions, and after that comes the first round of regular combat. This can be particularly nasty for characters like diviners, who always act in the surprise round, giving them one more action over everyone else because of their ability to glimpse into the future.
This is different from, but connected to, being caught flat-footed. According to page 567 of the Core Rulebook, a flat-footed character is one who has not yet acted in combat. They do not gain their dexterity modifier to their armor class nor can they make attacks of opportunity. It also makes you vulnerable to sneak attack. However, any character with Uncanny Dodge cannot be caught flat-footed, which makes barbarians, rogues, and others quite tricksy to ambush.
Rule #3: Acrobatics Can Make Fighting Defensively More Beneficial
Fighting on the defensive is a rule we don't usually invoke, but according to page 184 of the Core Rulebook you can choose to fight defensively. You take a -4 penalty on your attacks, but gain a +2 dodge bonus to your AC. However, as pointed out on page 90 of the same book, if you have 3 or more ranks in Acrobatics, you gain a +3 dodge bonus to your AC instead. If you take the total defense action, which normally grants you a +4 dodge bonus to your AC, you will instead gain a +6 dodge bonus to your AC.
Rule #4: Vital Strike is a Standard Action
The Vital Strike feat, which starts on page 136 of the Core Rulebook, are the bread and butter of many great weapon-wielding builds. In short, you take the attack action to make a single attack. If you hit, you roll your weapon damage dice as if you had hit twice (three times with Improved Vital Strike, four times with Greater Vital Strike, etc.). So if you are a level 7 barbarian, and you use your standard action to attack with your greatsword, you would roll 4d6 instead of 2d6 for your weapon damage.
That seems pretty straightforward, but it's important to remember that this feat can only be used with the attack action (which is the kind you use when you take a move action to reach the target, and then a standard action to attack). You cannot weave it into other special actions. You cannot, for example, use the charge action and Vital Strike at the end of it, because a charge is its a unique full-round action. You cannot use Spring Attack and Vital Strike on your target. Nor does Vital Strike have anything to do with the target's anatomy, despite the name. It is not related in any way to whether a creature is susceptible to critical hits, or if it has an alien anatomy. All you're doing is hitting it really hard, but we'd already named a different feat, Power Attack.
Rule #5: Sneak Attack Applies to Anything That Isn't Immune to Precision Damage
In the old days of Dungeons and Dragons 3.5, sneak attack had big blind spots. You couldn't use it on constructs, undead, plants, and dozens of other creature types. Unfortunately, a lot of players (and DMs) choose to use the rules they remember, rather than checking Pathfinder's update. Because unless a creature is specifically stated as immune to precision damage, such as oozes, incorporeal creatures (unless you have a ghost touch weapon), and elementals, you can still apply your sneak attack damage under the right circumstances.
So be sure your DM knows this rule, and always ask before you don't roll your bonus dice.
For more overlooked and misremembered rules, check out Playing By The Book: Some Pathfinder Rules Players Keep Forgetting over on Neal F. Litherland's blog Improved Initiative.
I am become death, destroyer of worlds.