What is wrong with this Phalaenopsis leaf?
 

This is one of my Phalaenopsis species - ID TAG LOST - Does anyone know what is going on with this leaf ?

This kind of Phal, with stems producing flowers for many months to years, likes high temperatures and humidity. It is susceptible to spider mites. I would check for spider mites. I would also wonder about physical damage - bumping or scraping the leaf, or sunburn.

Take a tissue, press it under a leaf near the stem, and run it to the leaf tip. With spider mites you will see brown or red streaks. They also often leave very fine cobwebs under the leaves.

Treatment is a stiff spray of water, 70% alcohol or dish soap solution (1 tsp /5ml in a quart /liter of water.) You need to treat every few days for 3-4 treatments to get them all.0

No signs of spider mites. I spoke with a friend of mine who grows orchids in his greenhouse - he said this Phal exhibits signs of sun damage. So I treated it with Epsom salts and moved it away from the window.

Finding out the cause or causes of issues with leaves etc will require enough information about the plant's environment - temperature (not just ambient temperature, but temperature of leaves, roots, media etc too) ----- humidity, air-movement, fertilising schedule, mag-cal schedule, media used, watering schedule, watering method, how long the orchid has been growing in that spot for etc.

Spider mites are indeed devastating on phals and lots of other orchids. Got to really watch out for them.

As for lighting levels ----- definitely that is where leaf temperature is a consideration. The light energy heats the leaves (the cells etc) and the heat can create issues with the health of the orchid if the levels are too much, or too much for too long.

WOW. Fascinating.

Different orchids need different light levels. Phals do well if sun rays never touch their leaves. If you put your hand between the light source and the Phal, and see only an indistinct shadow, that is enough for them.

All of my Phals are in front of a window but never receive direct rays of the sun on their leaves - so the jury is still out as to what those "yellow" spots are on my Phals leaf.

I assumed that it might have been due to sun energy - such as direct light heating up the leaf too much.

But - because we didn't know anything about how this orchid was actually being grown, only some comments could be made about what your friend mentioned - and also on your response to what your friend mentioned - such as moving away from the window, where it could have been assumed that direct sun might have been involved.

On the other hand, if the window gets very warm, and the heat transfers to the leaf, then that could be an effect as well. But - it's unclear as to whether other orchids are next door. So it really does help to describe conditions adequately - to efficiently get toward the source of issue.

Could later even possibly consider things like fungal activity ----- and any air-flow in the growing area - whether it gets humid in there with no air-flow etc. These aren't necessarily a cause for this case. Only considerations.

It could also be caused by splashing cool water on a warm leaf.

all valid possible reasons so I will just add that it could just be past stress signs. Or past sundamage, did this orchid always have this damage or did it develop recently?
Damage like this is permanent. It will get covered by the next leaf and be less noticeable but thats one annoying thing about phals, if they get nibbled on by spider mites the damage stays for years.