Incompetence / lack of common sense

Does anyone ever feel totally incompetent? Like they have no common sense. I've had 3 examples lately that have really gotten me down. They've all been at uni.

The first was when I was labelling our group assignment. I labelled my group assignment file as Tutorial 1. Except we were actually Tutorial 2. On my timetable it said "Tut1, 02" but Tut1 is just to indicate that it's a tutorial (if there was a second tutorial it would be "Tut2"), whereas 02 is the actual tutorial group number.

Second thing was an assignment asked for "hard copies of references". I originally thought they wanted us to print out each reference, even 40 page ones. But luckily I double checked and the staff said we just needed to print a reference list.

The third thing was that I was doing a peer assessment task on groupwork, and the sliding scales of how you rate each member changed e.g. if you voted one person up, the other votes for others go down. Not sure if that made sense, but anyway. Point being that I thought it was a program error and emailed coordinator. However he said that was intended and explained in the instruction.

These are just some examples. I've been plagued with a lack of common sense in many situations my whole life. Do others have similar experiences?

Plagued with a lack of common sense, by that you mean "cerebral flatulence", "pre-frontal a$$flap", "colon cologne of the cortex"...better known as a "brain fart"? Yep, I get those all the time, just recently here are a few of mine these past two months:
Pulling out of the driveway in my truck, "Why isn't it moving?" Oh...I forgot to put the truck in drive.
At work...I'm going to the back for something, I need a plastic bin, hey that girl's pretty hot...wait, what did I come back here for?
Get out of the car, I'm almost inside the house....I turn back, I forgot to close the car door.

I'm only 30 years old, is my short term memory shot to hell already?