1. not continuously making drones/scvs
2. waiting too long to expand (don't wait till your main is tapping out)
3. not spreading creep. At all.
4. getting badly supply blocked more than once in a game
5. queuing more than 2 units for building
6. Expanding then not anticipating the extra income and being caught short on production, then letting short term priorities still occupy your attention and not fixing it.
7. Rallying to a unit
8. Rallying via a long and unsafe path
9. Not having ONE army (unless you have a plan that needs it split)
10. Leaving 4 digit minerals / gas sitting in your current account
11. Having very unbalanced mineral/gas consumption/production (take some guys off gas if you have too much!)
12. (this is a huge one) not producing while moving your army forward for an engagement
13. Having no clue what the enemy is doing, or where his army is, or how many bases he is on
14. Ignoring xel-naga watch towers totally
15. Not retreating when the opposing army turns out to be too big
16. (opposite of 15) Not pressing on against a retreating army
17. Failing to realize when the other guy loses his army, he has nothing left and is probably only just starting production (see 12, see 16)
18. Hanging on for too long when you should gg (floating buildings off into corners and other cute but useless stuff)
19. Not recovering bunkers no longer needed, cancelling command centers being built & under attack
20. Walking along the bottom of (enemy) cliffs (esp with no air)
21. Having no plan or defense for cloaked enemy attack
22. Attacking with insufficient ability to atk air
23. Sending in your rushed single trophy unit (colossi, bc), with little to no support
24. Doing a ling rush to a blocked ramp and not retreating
25. Not getting an orbital command asap
26. Forget to scan when being attacked by cloaked units
27. Not having all your buildings in a control group, all your command centers in a control group
28. Not setting idle SCVs on auto repair near turrets or bunkers
29. (as T) army camping out in the wilderness and not building a couple of bunkers and putting a turret down
30. Not recognizing a harass as a harass (over-reacting)
31. Not lifting up your building when surrounded by ground force harass
32. Not understanding cliffs and visibility (in offense)
33. Letting tanks (or colossi) be in the front
34. Leaving tanks that can siege up, sit un-sieged
35. Going to three bases without the ability to defend an attack on one
36. Starting work on an expo without any static or moving defense
37. (as T) flying a command center to an island without first loading it with SCVs
38. Not checking you have 3 on gas especially after a fight at the mineral line
39. Using orbital energy to increase supply or do scans that could be scouted by a unit
40. Trying the same smooth move more than once
41. (as T) Switching cloak on and off, turning it on uses a lot of energy!
42. Having no clue how to use ghosts, ravens, and other odd units
43. When attacking, not being patient and attacking a less established expansion first
44. Forgetting to do tech upgrades
45. When flush with minerals forgetting you can build two research buildings and parallel research
46. (as T) Not getting stim, or getting it and not using it
47. When flush with minerals, not building out future supply
48. Screwing up your hot key groups, and not fixing it
49. Wasting time just watching the outcome of a battle (time passes really quickly!)
50. Wasting time watching a harass (a drop or whatever)
51. When in the enemy base, having no clear idea what buildings/pylons to kill as a priority (shooting at everything)
52. When attacking in confined area not moving your army up a bit more than they default - to give more units a chance to shoot.
53. Not decisively switching production when you get a best look at their composition and find they are one note
54. Not realizing rally points can have more than one leg to them
55. Not knowing how to queue commands to route and unload a drop of marines (doing it all manually)
56. Not being able to queue move and siege a tank
57. Rarely if ever making units follow other units
58. (as P) putting down too many photon canons
59. (as Z) operating out of one base for most of the first 20 minutes
60. (as T) going marine heavy against P in the late game (see 42)
61. (as T) not being aware that there is a P observer over your army, or base, or both
edit: and a free one, maybe the biggest
62. rarely looking at the mini map and not clicking in it
an after thought:
63. Rallying to your battle point, producing while moving, and forgetting you have a blocked ramp (coming home in tatters to see all the units that couldn't get out). Doh!