Did you know (so I've been told) that the maximum number of Friday the 13ths you can have in one year is 3? (I also think the minimum is 1.)
When I was in fourth grade (1983-4) we had three. Each time, the teacher put a ladder in the corridor leading to our classroom as an example of other superstitions. On each of the three occasions I decided to test whether or not walking under the ladder affected one's luck. What I determined was that walking under a ladder was fine, but if you made contact with the ladder you'd have problems. This was true whether you walked under or around it.
Now, when I say "problems" I mean it in a metaphoric way. Like having a bad day. But if you think about why people might have decided walking under ladders could be bad luck - perhaps due to the increased likelihood of things falling on your head - the ladder-contact corollary seems reasonable. After all, if you touch the ladder, things are much more likely to fall on your head than if the ladder is left alone. And if things fall on your head you're likely to have a very bad day indeed.