Sloppy coding has to be one of my main pet peeves. I hate looking at code that I have to pick up and maintain and realize that the person who came before didn't care one jot or tittle about the maintenance aspect. For hours at a time each day it seems like I look at code with huge blocks commented out. I don't think people understand that when using a source control system, YOU CAN GO BACK TO PREVIOUS VERSIONS!
It's not just the large blocks of commented code. I also deal with large amounts of poorly thought out code. Examples are people who use C++, but don't USE C++. Many times it seems like people just code in C, but name the file .cpp. What good does that do? What benefit is there? If you are going to use C++, use it for what it can be good for.
Then there are those who like to pretend they know about software development. They claim that their code is well written, that they follow good design principles. These people can have the worst code of all. They over comment, considering it "built-in documentation." If their code was well thought out, they wouldn't need as many comments, the code would comment itself.
The most frustrating thing is watching new projects head down the same road. I watch as people continue to put in hacks where a well thought out solution would do so much better in making the code maintainable and portable. Long down the road some poor sap like me will have to go through the code and wonder how it ever came to be.
I try and try to influence people to write good code, but alas the battle is never ending, uphill both ways in the snow.