But often enough I find myself in a situation where I have to back her - but don't know to what end. That places me into a situation where I am bound to fail.
To me that sounds like the rules that your wife sets a too complex. If you don't understand that they make sense as an adult, they will be completely arbitary to the children.
Insightful. The rules I am talking about are mostly about every-day routine. The grand picture of these rules is clear - but that doesn't help for particular cases where
a) the children may know the rules or rather the pragmatics of the rules better than I and
b) I don't know the 'usual' consequences with respect to rule violation.
This manifests especially if I care for the children only for a limited time and then my wife takes over again - and gets dissatisfied with the consequences of my decisions - mostly because she has to 'clean up' after my 'misapp...
Followup to: Strategic ignorance and plausible deniability
My in-law always says: "For children it is easier be forgiven then to get permission."
EDIT: This post is superseeded by my Book Review: Kazdin's The Everyday Parenting Toolkit I recommend reading only that. The remaining insight of this post is: Children expend more brain power on their parents than the parents on them.
I can say from experience: That is risky.
Children (esp. small ones) expend significantly more brain power on their parents than the parents on their children (your mileage may vary). I can assure you that they will notice these cases - at least some - and take that into account one way or the other.
If the children notice this they may assume that you either condone, accept, bear or ignore it. None of these has positive effects.
Possible alternative strategies:
I am influenced by The Adlerian School. Of relevance here is Striving for significance.
The testing of limits and the resulting interaction with the parent give the child a feeling of significance if the parent acknoledges the act of the child even if he doesn't agree with it. On the other hand ignoring the act of the child is negative feedback about significance.
EDIT: The asymmetry between parents and children with respect to the effectiveness of deniability can be generalized to any situation where one actor has significantly less overall information about the situation than another actor and thus might not be able to reliably estimate whether deniability is possible.
ADDED: tadamsmar pointed out that ignoring is scientifically known to be effective and the advice or rather personal expierence I have related in this post may be contraproductive (at least if applied in isolation).