Brown leaf

[Minimeben]

As I said before, new to bonsais, is this brown yellow leaf something to worry about?
Many thanks?

 

[cheap_walmart_art]

My guess would be thats somewhere an insect sucked on and opened up a pathway to some fungus. You should be able to remove that leaf and discard and be fine.

 

[Wires_Guy_wires]

I have foliage like that in the backyard, when the kids flick their burning doobies over the fence. I've been there, no hard feelings.
If there are smokers near, tell them to use an ashtray.

But this could just be fungal or mechanical damage even. The black part is where the plant has killed it's own cells either because they are damaged or infected. In the yellow part we see the plant draining the surrounding cells from nutrients to stop further spread of whatever started it. To a plant, it's hard to distinguish how those cells died, but the pattern afterwards is almost always the same whether it's a burning cigarette or an infection or some other type of damage.
If no other parts of the plant are affected, I'd just remove the leaf as cheap walmart art says. If they are affected, try to find a cause.

 

[leatherback]

Do not worry indeed.
However, on the leave in the background all the way on the right it looks like it could be infected by probably thrips or mites. That I would check as this can become a problem.