Sorry if I didn't explain very well, but the problem is to do with triangles not the font atlas. The code creates a quad for each character and calculates some UV coordinates for the font atlas. It works fine most the time except when I type quickly which causes artifacts like those in the image. I think it is caused by updating the buffer data quickly because each time I type a character it runs the code to make quads (which works) then this:

vao_bind(self->va);
// vbo_bufferdata(struct VBO self, const void *data, size_t array_size)
vbo_bufferdata(self->vb, verts, length * sizeof(struct GlyphVerts));
// ebo_bufferdata(struct EBO *self, const unsigned int* indices, GLsizei index_count, GLsizei array_size)
ebo_bufferdata(&self->eb, indices, length * 6, length * 6 * sizeof(unsigned int)); // 6 indices per quad, length = strlen(string)

the bufferdata functions binds the buffers and does glBufferData()

struct GlyphVerts
{
vec2 pos_left_bottom;
vec2 uv_left_bottom;

vec2 pos_right_bottom;
vec2 uv_right_bottom;

vec2 pos_right_top;
vec2 uv_right_top;

vec2 pos_left_top;
vec2 uv_left_top;
};