I found a bug when you copy-paste sprites from one cap to another. This post is just for people who already (i've read those advices about buggy copy-paste from cap to cap) copy-pasted their objects between caps. This is a road too nowhere.
Known symptom - when you duplicating one of the sprites (i mean creating another instance of it on layout), the other sprite (the other object) disappears from the object bar, but in the project you can see it in it's folder, and all events that involves it are there too.
Maybe you all already know about it and not doing this mistake, but for those who have their backup that is more or less up to date, i suggest to roll back before it's too late.
P.S. Sorry if my topic will be considered by someone as flood, maybe somebody will find it usefull.