1. Pedal is reversed. Some organs have a swell pedal which opens the shutters when we press it with the heel (contrary to the most which opens when we press with the toe).
Here you have to consciously think about this difference from the swell pedal and even mark the opposite motions in the score. Practice the opposite motion ahead of time on your organ.
2. Pedal is too far right. Normally swell pedals are positioned between the D# and F# in the tenor octave. But some organs have the swell pedal which is positioned on the extreme right of the pedal board.
Try to sit on the edge of the bench which will help you reach the swell pedal easier. Lean forward to have a good balance.
3. Pedal is too heavy/too light to operate. With some swell pedals you have to use much force to open it. And the opposite is sometimes true - as soon as you touch it - it might open without you even realizing it.
In both cases you have to consciously think about this feeling ahead of time and make a mental note to use more or less force.
Next time when you play an organ with a swell pedal, try to find out whether these above problems are true and if yes, consider them in your practice or performance. It's best that you find out what kind of swell pedal and how it operates well in advance by asking about it the local organist. Then you can be sure that you will have ample time to adjust and prepare yourself to the new playing experience without too much stress.