Oh boy. I've given myself a challenge now.

[Peter_in_2780]

As h-t-c said, an off-by-one. Or an extra semicolon on an if() or for() - can make the point that the compiler doesn't give a toss about your pretty indentation when determining control flow.

Software rusts. Simon Stephenson, ca 1994. So does this signature. me, 2012

[Dave Kreskowiak]

So you're writing a book? 'cause that's what ends up happening when you go down this rabbit hole.

Asking questions is a skill CodeProject Forum Guidelines Google: C# How to debug code Seriously, go read these articles.
Dave Kreskowiak

[Amarnath S]

One more - Missing variable initialization

[Lost User]

How about NullPointerException? That seems to be one that so many people do not even understand what it's about.

[Lost User]

Two different variables, with much too similar names - and then using the wrong one. My_Increment My_Decrement

[dan sh]

Something like this. Includes missing out on floor, ceiling and index and more. // Percentile calculator List values = new List { 1, 2, 3, 4, 5, 6 }; double percentileToFind = 0.90; // find 90% of 6 since there are 6 numbers int indexToLook = (int)Math.Round(values.Count * percentileToFind); double startingValue = values[indexToLook]; // Omitting rest of percentile calculation logic // 95th percentile percentileToFind = 0.95; // find 95% of 6 since there are 6 numbers indexToLook = (int)Math.Round(values.Count * percentileToFind); startingValue = values[indexToLook];

"It is easy to decipher extraterrestrial signals after deciphering Javascript and VB6 themselves.", ISanti[^]

[PIEBALDconsult]

Using the debugger is an admission of defeat. A option of last resort (just short of actually asking for help).

[kmoorevs]

Division by 0, array index out of bounds, null values? :)

"Go forth into the source" - Neal Morse "Hope is contagious"

[Matias Lopez]

var vAux = 'whatever...';
if (vAux = null && vAux = undefined && vAux.length = 0)
console.log('Error at ValidationCode!');

[Matthew Dennis]

I think one point that is often missed in teaching coding is making sure you can show that your code is correct. Not necessarily TDD, but setting up some input that you know what the output should be. This is very helpful in understanding the problem, and helps you know when you are done.

"Time flies like an arrow. Fruit flies like a banana."

[DRHuff]

Cut and paste errors x = x + 1; y = x + 1; (Slightly more complicated would be better) Because your brain sees what it wants. Not what is there! (My sons first comp sci program. He didn’t see it. I didn’t see it. but stepping through the debugger sure made it obvious!)

If you can't laugh at yourself - ask me and I will do it for you.

[OriginalGriff]

I'd agree - and one of the planned articles is all about testing. I'll be mentioning it in "Debugging" but not to any depth.

"I have no idea what I did, but I'm taking full credit for it." - ThisOldTony "Common sense is so rare these days, it should be classified as a super power" - Random T-shirt AntiTwitter: @DalekDave is now a follower!

[k5054]

Infinite or range-error loops e.g while(test = 1) or for(i = 100; i > 0; ++i) I know I'm guilty of both of these, on occasion.

Keep Calm and Carry On

[kmoorevs]

k5054 wrote:

Infinite

:thumbsup: and put a short counter on it so that it doesn't completely blow up! :laugh:

"Go forth into the source" - Neal Morse "Hope is contagious"

[Lost User]

We / I learned how to flowchart before learning to write code. Don't know how you get to one without the other (at some point), without a lot of "explaining".

"Before entering on an understanding, I have meditated for a long time, and have foreseen what might happen. It is not genius which reveals to me suddenly, secretly, what I have to say or to do in a circumstance unexpected by other people; it is reflection, it is meditation." - Napoleon I

[trønderen]

Comparing for equality two real valued calculated results, which should mathematically be identical, but due to limited precision, they differ in the least significant bit (or two). If you were working on old machines using 1-complement integer representation, you could also compare plus and minus zero for equality, but I haven't seen a 1-complement machine for quite a few years now. A related one: When casting (implied or explicitly) to a longer word length, will the upper bits be zero or sign bit filled? Note: This can be both language/datatype and machine architecture dependent. I once spent half a day to understand a single if(x>y) statement: Both alternative seemed to do exactly the same, just in slightly different order. The reason was an implied cast (from 8 to 16 bits - this was on an 8051) where "wrong" sign extension would wreck the result if the same order of execution was used in both cases. Edit: This was intended as a reply to the OP, not as a reply to Gerry.

[charles henington]

Using iterative instead of declarative statements to manipulate collections using Linq??

[Mycroft Holmes]

I'm not sure this can be done anymore, declare a global variable DateTime as an int. Worked in VB (possibly 6) and truly screwed up an application.

Never underestimate the power of human stupidity - RAH I'm old. I know stuff - JSOP

[FreedMalloc]

I was once told to write a short snippet in C++ with a couple bugs in it. One obvious and one less obvious to give as a coding test to prospective new hires. It took me hours. Plus, I never docked a candidate that "failed" the test in an interview. And thus hold interview tests in the highest disdain. My suggestion: An if statement with 2 indented lines under the conditional but no enclosing code block delimiters (braces).

if (x > y)
x++;
y++;

[englebart]

Integer arithmetic 3 / 2 Integer versus double If 3/2 == 1.5 Another post covers double == double. Always use code formatter to help with misleading indents/ missing braces.