Nothing............only you will get 25hz frequency at out 
put.

It will trip on over fluxing protection, if the protection 
is installed.

It's not a wise situation to connect transformer to a lower 
frequency with same voltage. The flux induced in core is 
directly proportional to voltage & inversly proportional to 
frequency. Hence the core will be over heated/over fluxed 
and may get damage due to over heat.

The only remedy is reducing the voltage in same ratio.

flux is proportional to E\f or V\f
if the frequency is reduced the ratio V\f will increase so 
flux will increase.so to maintain this flux magnetizing 
current required will increase.this reduces the no load 
power factor , so on load power factor is also reduced.
this also reduces o\p of transformer.so V/f ratio of t/f 
must be maintained constant.
as 50% of frequency is applied to maintain constant V/f 
input voltage applied must be 50%if rated voltage