Here are 20 reasons, off the top of my head:
DISCLAIMER: These don't apply to FM/IM/GM, obviously.
1. You repeatedly state that "chess is art", and to prove this, you post your blitz miniatures for analysis and "to help others learn".
2. You legitimately think that you have a "style".
2a. You think that you play like Morphy or Tal when in reality you just fling pieces at your opponent as you don't know how to do anything else.
3. You think that you are severely underrated.
4. You play the Poisoned Pawn Najdorf and the Botvinnik Semi-Slav because a book told you a line that "guarantees -0.6".
5. You frequently mention how much you study to anyone who will listen.
6. You think that you've invented a new line or discovered a TN, and you name it creatively and then tell everyone who will listen.
7. You blame your openings for your losses.
8. You think that "experience is the best teacher" and "study" by playing copious amounts of online blitz.
9. You have at some point asked a titled player for an autograph.
10. You frown upon anyone who plays conventional chess as "uninventive" and attempt to drag your "boring" opponents off theory and "into REAL chess" by playing garbage instead of logical moves.
11. You look at any rating much higher than yours as either unattainable with anything but tremendous talent or easy to obtain if you bothered.
12. You think that your advice is more valuable than an IM/GM's for coaching as "you know what it's like" to be at their level (yes, this holds true for candidate masters who are misguided).
13. You "refute" books like My System or My 60 Memorable games with Houdini.
14. You don't know the difference between positional and passive.
14a. You play a passive system like the London and think that it automatically makes you a "positional" player.
15. You ask for concrete answers on questions that simply can't be answered (especially "what opening should I play"?)
16. You devote far too much attention to your opponent's ideas instead of your own and end up responding to him instead of playing for yourself.
18. You own Dvoretsky's Endgame Manual, Fundamental Chess Endings(Muller), Endgame Strategy(Shereshevsky), How to Play Chess Endings(Muller/Pajeken), and Rook Endings (Smyslov+Levenfish) and frequently talk about them, but have only read a page of each and couldn't understand any of it.
19. You are offended by any of the previous notes on this list.
20. You didn't notice that there is no #17, and you spend your time making arbitrary comparisons of chess players from different time periods instead of studying.
When you realise that black moves first