But everything good has its limits. If you use Labels enthusiastically, as I have, you will discover them (they will find you) periodically.
You have too many labels, or the total length of your labels is too great.
You are limited to a total of 200 characters, in the Labels string, for any post. I believe that this includes all commas and spaces between labels.
Look at my example.
Labels for this post:
Label1, Label2, Label3, Label4, Label5, Label6, Label7, Label8, Label9, Label10, Label11, Label12, Label13, Label14, Label15, Label16, Label17, Label18, Label19, Label20, Label21, Label22, Label23, Label24, Label25, Label26, Label27, Label28, Label29, Label30, Label31, Label32, Label33, Label34, Label35, Label36, Label37, Label38, Label39, Label40, Label41, Label42, Label43, Label44, Label45, Label46, Label47, Label48, Label49, Label50, Label51, Label52, Label53, Label54, Label55, Label56, Label57, Label58, Label59
Like limiting the size of your main page, you really have to exercise some judgement about labels. A blog of any appreciable size, with 200 characters of labels in some posts, would have a rather large label list. The above example, with 59 labels, is waaay to many. Think of your readers, please.
(Update 2009/10/02): Nearly one year later, and we see newly discovered limits in labels.