Answered by Sidi Mostafa Azzam
Is it valid to follow someone who might do something not considered valid in any madhab? (i.e. wiping over cotton socks or invalidly combining)
The Position of the Shafi`i School
According to the Shafi`i school, it is not valid to pray behind someone whose prayer is invalid according to the Shafi`i school, even if he is not Shafi`i, and even if it is valid according to another school. This is the position of the school.
A Weaker Position
There exists another position in the school that is weaker (but is strong enough to follow in one’s own personal life) that it is valid to pray behind an imam whose prayer you consider invalid so long as he considers his prayer valid. Following this position allows you to pray behind many you would otherwise be unable to pray behind; for example, someone that wipes over socks that are not water-repellant, but are strong enough to walk around in.
Praying Behind Someone Whose Prayer MIGHT Be Invalid
As for someone whose prayer might be invalid, what I have learned is that the assumption on the imam’s prayer is that it is valid … unless you are aware of something he is doing that invalidates. One need not assume that his prayer is invalid just because it might be.
And Allah knows best.