But everything good has its limits. If you use Labels enthusiastically, as I have, you will discover them (they will find you) periodically.
You may see the advice, when editing your post.
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. Apparently, 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.