Some t-shirts are funny, and other t-shirts are so painfully un-funny that they're also funny. The list below is dedicated to the latter.
10. More Cowbell - Is it a requirement that every aspiring t-shirt designer include a "more cowbell" design of some sort? Yes, the skit was funny, and yes, Christopher Walken is a god, but does the world really need another t-shirt to commemorate the clip?

9. Don't Taze Me Bro - Okay, we all knew it was coming. Unfortunately, this is just another one of those things that's going to be beaten into the ground until even the original is no longer funny (Chapelle's Rick James line, anyone?). The designer here has clearly put a lot of thought into his design, though. Note the "clever" lightning bolts over the word "taze". Genius!

8. Sore Loserman - This might have been funny back in 2000, but I doubt it. If you're wearing this shirt in 2007, you need to re-evaluate your life. It was 7 years ago, move on already.

7. Chilltown - It is 99.9% likely that a man who wears this shirt starts every speech with a quote or definition out of the dictionary. He frequently ends conversations with the "wink and the gun", and he wishes he could just find a nice girl like dear old mom.

6. It's All About Me - I used to know of a very large, outspoken trailer park woman who wore a shirt with the same message. Come to think of it, this shirt and her "Next mood swing in 5 minutes" shirt are the only two shirts I ever saw her wear. I guess I shouldn't fault the designer, since someone out there is obviously buying it, but this shirt provides a bittersweet reminder of the fact that there are definitely people in the world who are even less cool than I am.

5. I'm the mechanical engineer your mother warned you about - My mother warned me about a lot of things, but ruler-wielding engineering nerds weren't among them.

4. S=ex^2 - If you're cool, you're not going to be caught dead wearing this scientific sex equation. If you're the kind of nerd who would wear it, you're probably just irritated that the letters don't stand for anything in particular.

3. Caffeine Addict - I challenge you to find an office with more than 10 employees where no one has posted a trite caffeine comic strip, sign, or joke in some part of the office. The designer of this shirt is just contributing to the surplus of tired caffeine humor.

2. Maine Coon for Halloween - To say that I'm an animal lover is a gross understatement. All the same, shirts like this one make me cringe. A cat in a Halloween costume might be funny. I wouldn't wear it, but we'll let it slide. A cat in a Halloween costume with 3 lines of explanatory text just misses the mark.

1. Ninjas Are Cool - Apparently, these guys are not familiar with the way the "Something is X. And by X, I mean Y" joke works. You're supposed to follow up with something that (a) means the opposite of the original statement, or (b) reveals something surprising and humorous.

Examples:
Ninjas are stupid. And by stupid, I mean totally sweet. (Note that this is STILL not particularly funny.)
I'm a ninja. And by ninja, I mean downtrodden office worker who occasionally throws a few air kicks after indulging in a Sunday afternoon Michael Dudikoff marathon. (A little long for a t-shirt, perhaps, but you get the point).
Reflection:
I have to wonder - would the t-shirts on Cafepress be this bad if they charged designers a small fee for each design created? Are the designers really this un-funny, or are they playing a numbers game, knowing that if they create enough bad t-shirts, someone will buy?


The ninja shirt is a reference to this site:
http://www.realultimatepower.net/
Facts:
1. Ninjas are mammals.
2. Ninjas fight ALL the time.
3. The purpose of the ninja is to flip out and kill people.
I wish I had a mechanical engineer to buy that shirt for.