What happens if one fails in the 1st MB in WBUHS?

If you fail the 1st MB examination in WBUHS, you will need to sit for the supplementary examination, after a gap of 3-6 months. If you pass that, you can continue studying for the next year. If you fail then, you will have to repeat your first year again with the next batch. 

But you can easily clear your supplement and also prepare for the 2nd MB examination with some effort.