Here's a pet peeve of mine: the omission of commas at the ends of lines of verse.
Here's the example I just came across, from "Cher," by Dorianne Laux, today's poem in the Poetry Calendar 2007:
I wanted to wear a lantern
for a hat, a cabbage, a piñata
and walk in thigh high boots
Am I just being too picky (too much of an English teacher) when I stumble over the absence of a comma after "piñata"? Otherwise, the poem uses conventional punctuation throughout, and no other commas are omitted at the ends of lines.
I don't want to pick on Laux about this; it's something I see quite often, and I frequently stumble over it. In Laux's poem, at least, the missing comma does not make the grammar and syntax confusing, or create a distracting uncertainty about what exactly is meant (not an ambiguity but an uncertainty, which is a different thing).