Even if you thought out all the details, and everything should be alright, any casualties can happen.
For example, you can forget that it has recently been raining, and rains have affected the groundwater level. So, if you want to use the underground passage, you will fail. It seems to be dry in front of the tunnel and to be safe to go through it. However the passage goes deeper. Brave-hearts can venture to swim underground in conditions of a limited oxygen amount, but they do not manage to escape...
Don't worry, the next time you probably can use this way to cross the border if the water-level become normal. However it may be raining again... That's why you should always monitor the weather prediction!