Incautious use of semicolons

Be careful with semicolons. Following example

if (x > a);
   a = x;

actually means:

if (x > a) {}
a = x;

which means x will be assigned to a in any case, which might not be what you wanted originally.

Sometimes, missing a semicolon will also cause an unnoticeable problem:

if (i < 0) 
    return
day = date[0];
hour = date[1];
minute = date[2];

The semicolon behind return is missed, so day=date[0] will be returned.

One technique to avoid this and similar problems is to always use braces on multi-line conditionals and loops. For example:

if (x > a) {
    a = x;
}