I’ve posted about this before but, there is an annoying little bug in blender that makes all object either invisible or visible seemingly at random, so I was wondering if there is a key-bind to toggle the visibility of any selected objects.
it is a huge pain in the A…
I don’t know how to mark all visible…
Are you using Occlusion Culling, because I recall that it’s practically unusable for large levels because of a bug like this.
If so, then you will need to stop using it and make your own system.
If it’s during general work in Blender though (when the game is not running), then this is the wrong section for this type of thread.
I usually only hang around the game engine sub-forums, so where should I post it?
I’d say either the “basics and interface” forum or the “technical issues” forum (this sounds closer like it would fit in the latter), you can then get help to confirm if it’s a genuine Blender usage bug.
Okay, thanks. Also is it the technical support forum?
no, its a shortcut, near ctr+f and it happens to me when trying to go into cam fly mode occasionally
Yes.
BPR; your issue then isn’t specifically related to the BGE, but related to general Blender usage.
Do you know what shortcut it is?
W key 
press W -> clear all restrict render
I don’t know about you but, I’m unbinding that key ASAP.
‘h’ hides selected objects
alt + ‘h’ unhides all hidden objects.