I payed for Ravens in the Fusion Core. But they did not spawn, because I did not have enough supply. I tried to cancel building them, but the minerals and gas were not refunded. So, I tried to buy the Ravens again (but I had no mins and gas left)... surprisengly, I bought them again with no cost at all, the same number (but not extra - I did not have enough mins and gas left after buying Ravens the previous time).
They spawned later, when some of my units were destroyed and I got some empty room in supplies.
They spawned later, when some of my units were destroyed and I got some empty room in supplies.