What's Wrong With This Code

What's wrong with this code, which uses a second thread to build new nodes in a scene graph while the main thread renders and periodically picks up finished work?
thread 2:
create mesh geo in memory
glBufferData to create a vbo
send msg to thread 1 with VBO ID
thread 1:
while(1)
{
if we get a msg
insert VBO into scene graph
draw scene graph
}
Or how about this logic, which uses threads and PBOs to asynchronously capture full screen to disk?

thread 1:
render frame
bind pbo
glReadPixels
unbind pbo
send msg to thread 2

thread 2:
wait for message
bind pbo
get pbo data
unbind pbo
write data to disk

Hint: the answer is the same!