ruining the surprise for one player...

The players locked themselves in the basement with the cauldron.   As they discussed their day and stuff, a sneaky surprise, sneaked up on them.  All except for one player, that shouted out, "What is with creepy Ghimli sneak a sneaking", I had checked all of their player visions (ctrl+l), and everyone was confined to the room.  The player could see everything beyond the wall.  Refreshing the browser, fixed the vision glitch, but really the surprise was spoiled after that. Not even sure how to report this, as I have no control over the players system and don't know the browser/OS (guessing @ chrome/windows10)

Hi DMJamesO,  One of the problems with Dynamic Lighting is that when it fails for a user, they see everything because it is put on top of what they would otherwise see.  It is my experience that the first fail point is Explorer Mode, so I would try disabling that if you are using it and see if that prevents their failing to see it.    Second, you might want to try permanent darkness as a control method. It might be that Permanent Darkness is retained even when Dynamic Lighting fails, but I am uncertain about that.  If neither of these solutions work, you may want to switch to Fog of War rather than use Dynamic Lighting. It is sometimes the case that a machine cannot handle Dynamic Lighting.  Additionally, consider placing tokens on the GM layer, even if it is supposed to be in an area where they cannot see it.  As for filing a bug report with the Devs, the best place to contact the Devs is via the Help Center  as the bug report forum is primarily for users helping users. Your player will need to help with that by providing information.  Some things you can have your player check though:  Are they using Chrome or Firefox? If not, they should be using one of those two browsers with Roll20.  Is their Hardware Acceleration turned on? If not, have them turn it on.  Here are some troubleshooting steps for graphics issues with Roll20.