Stuff that annoys you...

when reading code not written by yourself. For example, I cannot stand it when  people use

if (condition)
    ...
else if (condition)
    ...
else if (condition)
    ...
else if (condition)
    ...
else

instead of the much more readable


if (condition)
    ...
else
    if (condition)
        ...
    else
        if (condition)
            ...
        else
            if (condition)
                ...
            else

>>1
I prefer (1) where respectable alternatives do not exist or are not as useful.  (2) produces tree-like code if you nest if else statements in each other and that could affect how the code is displayed by your text editor or IDE (heaven help you if you use an IDE).  The "else if" conditional structure is an established feature in most languages.