Inspect the wood that you are working with. It can have imperfections that can cause it to kick back, get stuck or bind on the blade, etc. It is important to inspect your workpiece before you do any cutting or machining.
If you work with some reclaimed timber this is even more important because it can conceal nails, bolts and other objects. You can usually cut through nails, but it may damage the blade or the router bit. I would not recommend it. You can normally see these objects sticking out on the side of the timber, but sometimes it is concealed deep into the center of the plank. Get yourself a cheap metal detector from any good hardware store, they are not that expensive and easy to operate and understand. Spending R200 to R300 will be a good investment that you will be glad you made.
The moisture content in timber can also be a factor of timber binding to the blade, especially when ripping planks lengthwise, the thicker the piece, the bigger the problem. I recommend buying a inexpensive moisture meter, also from you local hardware. It will not only limit accidents but also help with the quality of your end product, wet timber is a no-no. The acceptable moisture content inland in South Africa is 12% and less, obviously it will be higher at coastal areas and areas with a sub-tropical climate.
Knots in timber usually do not create issues if you are patient and prepared for the cutting to take longer. Just keep all blades sharp and you should not have problems. Loose knots may be a issue, so remove them before cutting or machining.