What happens if I connect a transformer in reverse?

As Mr Banana says - magic smoke happens. Because ...

Energy is transferred in transformer via a magnetic field. The field is produced by the amp-turns in the core (amps flowing x number of turns). Above a certain level the core cannot support any more amp turns and the core "saturates". What was an inductor with resistance to AC of far more than its resistance becomes mainly a resistor.

You'd get lots and lots and lots of amps in the case that you mentioned - so much so that if the fuse didn't get there first the transformer would DEFINITELY be destroyed.

The iron core in a transformer is usually operated on the part of its magnetic curve where it is beginning to saturate and get less efficient. This is to get as much use of the steel core as possible. They are run close enough to "the edge" that a transformer made to run on 60 Hz mains will get much warmer on 50 Hz mains at the same voltage as the cycles are 60/50 = 20% longer and the current in the winding gets that much longer to increase and ...

So a SLIGHT overvoltage may work OK - say about 20% max. But 230/7.5 0 30+ times as much "will not work"! :-)

Just try it. Just kidding, don't. Inductive reactance is the key. The 7.5 volt winding will smoke at 250 vac, might even spontaneously self-rapid disassemble.