Even after all this time, I still sometimes inadvertently use assignment instead of testing for numeric equality:

if ($first = $second) {
  # do something
}
else {
  # this code is never executed since assignment to $first always retu
+rns true
}
[download]

Sigh. It is dumb, but I did it just last week.

Update: as pointed out by jdporter, in cases where $second evaluates as false, the resulting assignment to $first will also return false. While this rarely happens to me (since I'm usually trying to evaluate numerical equality between non-zero variables), it is an important distinction.