A group of people took an online webinar yesterday. The instructor is a heavy Adjustment Brush user, in LightRoom. After taking the class, many of the students in the group are reporting the appearance of weird blocks on images, after using the Adjustment Brush - both in LR and in ACR. It is also happening to me. I am a heavy Photoshop and ACR user - never before have I seen this. The only thing I'm doing differently is using the Adjustment Brush (because of the webinar class, as it was part of the instructor's core teachings). What could be causing this?
There was an adjustment brush bug in older versions of ACR. I first reported it here, and Eric Chan acknowledged the problem. It went away in future releases. Sorry, I can't remember what version numbers were involved.
(see youtube demo)