Why small mistakes are disastrous?

I made a stupid mistake. very stupid one!

if(!IsOK())
   OkDoIt();
else
   NoWait();

Do you see the problem?
Yes, only one character ruined the whole process.

It should be like this:

if(IsOK())  // if(!IsOK())
   OkDoIt();
else
   NoWait();

If the function containing this problem is rarely called, it's a time bomb inside a briefcase. Even after you recognize something is wrong, it's very hard to find this kind of mistakes because it's such a small one like off-by-one error. almost invisible. That's why small mistakes can be much more disastrous than big ones which are obviously more visible.

Since code like the above is usually written very quickly and unconsciously without thinking, if you don't have right habits, you will make a mistake. It brings me the idea I loved but, apparently, forgot.

Good programmers have good programming habits.

I will never use ! in if statement. ever.

One more thing to remember.

Test every changes you made.Every single line.

Of course, I tested my changes before I checked in it. However, there were many changes and I just quickly checked a normal flow without checking all the changes I made. I was focusing on bigger and more complicated changes. I should have kept my changes small and checked in them more frequently.

Anyway, it's absolutely my fault and I will never forget it.

I really don't think anything I do is a mistake. It could be if I didn't learn from it.

- Fiona Apple