I would first question how the plant could have gotten a sunburn on only this spot/leaf. If the window lighting somehow fell more strongly here, I would think sunburn. If I had a few other leaves with some irregular brown spots that were more dry than wet, I would think sunburn. If I had splashed water which may have heated up, I would think burn again.
If this is kinda outta the blue...I would think Erwinia.
Regardless, if I have a single bad looking leaf, I get rid of it.

Cut this leaf off with a sterile blade at least one inch into good tissue. Consider treatment with an anti-baterial like Phyton 27; but most importantly, increase your circulation around your plants.