Is it intentional or a bug that Fill 'er Up! skill first increases SP by 50, then takes -1 SP as the cost of the skill instead of first consuming the 1 SP it needs, then increasing SP by 50? You cannot really see this other than when you try to get SP that meets or exceeds your maximum SP, you end up with max-1 SP. The only explanation for never getting the SP filled up but always lagging 1 behind max is that the cost of 1 SP is actually deducted after the 50 SP has been given.

Now if it is intentional, then one should be able to cast Fill 'er Up! even when having 0 SP, since the 1 SP is taken away after the 50 SP is given

I think there's an off-by-one error lurking in the effects code somewhere, because that definitely is supposed to be: "spend 1 SP, then gain 50 SP". We've seen this sort of off-by-one reported elsewhere, so my feeling is that this is exactly the same thing at work.