Hi There.
I have looked around but could not see this bug posted.
I was playing online with a friend and we both went into minus metals and crystals when buying research.
However we could still continue buying more research, even tho we were in minus resources (except credits) which qued itself up and then activated. (I don't know if this also worked if buying structures, but i think it may have for ships)
(Added #1) I think it was about -500 for crystal and -300 for metal max (could be wrong tho).
(Added #2) If i can remember correctly i could also buy resources and it would go up, (i remember that bit) but i could also sell recources while still in negative - (if memory serves me correctly). (It basically acted like i had the resources - up to a limit, but just had a minus sign in front of it).
I have a screenshot somewhere knocking about and will try and post it if it has not been seen before.
(My apologies) it seems to have been already found here:
https://forums.sinsofasolarempire.com/333464
(I believe it is caused when trying to upgrade a starbase on multiplayer which dosn't work and then you cancel it).