This is two bugs in one:
- Somehow, I managed twice to transfer aircrafts between a production base and a destination base, with room for them (if I remember well, based on screenshots), but the Transfer screen tells that there's a capacity overload. In both cases, a hangar was under construction, perhaps quite near completion).
- The transfer system being so fooled, prevents any other transfer to this destination base, whereas the base's stores are not full. This means that while the scheduled craft has not arrived, transferring an ammo clip is not possible! (The same thing occurs when the Alien Containment Unit is overloaded, btw).

==== Expected behavior (or not?): ====
# If no free hangar, no craft can be bought nor transfered to the base.
# If a craft is being transfered (transiting to new base), it's listed in the base's craft roster, and considered as occupying its future hangar.
# Should the hangar be canceled or destroyed, the assigned craft should be either lost, or proposed to be sold or transfered to a new base. At once. If the assigned craft is still transiting, then something should be done (but it's not the topic here): e.g. the same Sell/Transfer choice could be proposed when it arrives.
# Should a base be overloaded as far as a transfer category is concerned (here: craft): the game pauses until the problem is handled (isn't this currently the case with the "aliens" category?).

==== Current behavior: ====
# This works as expected. What now if a hangar is under construction at destination base?
# The transfered craft is listed in the base's craft roster, and its hangar is counted as occupied in the Buy/Sell screen (e.g. 0/3 free). But does another hangar under construction interfere with the base capacity computation, or what does mean the e.g. -1/3 free in the Transfer screen mean (same situation)?
# I don't know.
# The game goes on. Any transfer from any base to the destination base is prevented.


Perhaps, the (-1/3) display is only a way to tells that a fourth hangar is under construction, and that the base's capacity will be soon (1/4). In this case, the transfer system shouldn't detect a base capacity overload.
In case this might be the same for any base container, such as employees and Quarters, or items and Stores, a check could be worth here.