Design of tables with large optional fields?

I have a general SQL design-type question.

I want to log errors to a table. If the error is with a URL, I want to store the URL. These URLs can be very large, hundreds of characters, but I only need to store it if it causes the error, which should be very infrequent. Which is the better design:

1. Create a large varchar field in the log table to hold the URL, or null if the error wasn't with the URL.
2. Create a foreign key field in the log table to a second URL table, which has a unique ID and a large varchar, and only create a record in this table if the error is with the URL.

One concern I have with design 2 is that there could be many other fields that are infrequent. Do I create a separate table for every one

Richard