Training with Okinawan weapons can be dangerous to the practitioner, others in the dojo, and the dojo itself. Therefore, special care must be taken whenever the student practices with weapons.
The student should start out slow when practicing weapons and make sure they are far enough away from their training partners and their surroundings so as to not hit anyone or anything as they practice.
It's easy to hurt yourself with when training with weapons.
One story that's kind of funny is that I made my own nunchaku as a teenager when I first started out in Karate. I was very excited to try to learn to use 2 sets of Nunchaku at the same time. I was in my front yard practicing one day when I decided to try swinging the nunchaku in opposite directions: one high and one low. I was sucessful a couple times but on the third try, I hit myself in the head with one and in the knee with the other, simultaneously. In my moment of pain, I heard someone laughing and looked up to see the paper boy having a good laugh at my expense. Looking back, I probably should have done something to pad the nunchaku before trying this stunt. Nowadays, you can buy padded nunchaku. They are a good investment :) The other thing I should have done was practice somewhere out of site to save myself from humiliation.
I have seen or heard stories of many people hurting themselves with Sai or coming close to hurting others while practicing. One such story involved someone letting go of a sai during a test and the sai whizzed past someone's head and stuck in the wall. It's very easy for beginners to drop sai while trying to flip them. Can you imagine a sai through the top of your foot? Ouch! Sai also have a tendency to get stuck in the sleaves of your Gi top. It's good to practice sai in a t-shirt for this reason.
By far, the worst stories of self injury come from Kama. The kama we use are very sharp and can easily do much self damage. This is why I will only teach Kama to black belts. I remember one student, years ago, who stuck a kama in his leg just above the knee. Another, cut the end of his finger off. The worst, was when someone layed their tricep muscle open and had to have 30+ stitches. Kama are very dangerous. They should not be practiced alone and they should not be practiced by beginners.
Bo can be dangerous when practicing in a group because of their range. It's hard for beginners to judge the length of the bo. For this reason, students should space themselves as far apart as possible when they are first learning. It's harder to hurt yourself with a bo, but not impossible.
The student also needs to be careful not to damage the dojo when practicing weapons. In our dojo, for instance, we practice on modern vinyl tatami mats. A Sai point or the end of a bo could easy ruin a perfectly good tatamai mat. Also, a bo through the wall or window would be an issue. For this reason, beginners should start out slow.
Learning kobudo as a traditional martial art is a great supplement to karate training, but be careful, and always learn under an experienced instructor.