Love to assist but your picture is really dark making it somewhat difficult to respond.

I lightened the original up a bit to have a look. It looks okay. The oldest leaves eventually do dry up. How long have you had it? Might be headed for etiolation; give it as much light as you can.

@ bikerdoc5968 , sory. Thanks anyway.

@hoovb, thank you for taking the time to do that, and for the advice. I understand that they do that, but Iôm concerned about how often it happens. They are in a well lighted room, but still a couple of them are etiolating, true.

I uploaded a new picture, from a different plant (actually, a "baby" growing) that has the same brown thing.

No problem. I'm sorry I didn't thing to Photoshop your picture. The new picture appears to have some physical injure that has caused scaring to the two leaves. This happened when the were very young and just erupting from the meristem.

Now I see your problem. I didn't see it, even on the pic Hoov lightened for us (thanks!) before. I believe it is caused by insects (deep in the spaces between leaves, as it's growing out (as Howard explained). Maybe a spray w/50/50 rubbing alcohol/water is in order. Check all plants.

Scaring? Well, at least it seems I'm not over/under-watering. I see a fruit fly now and then around the plants, but I never thought they could do that. I'll check nothing else is creeping on the soil or similar.

Thank you very much to all of you guys.