What is the word used for something that becomes redundant immediately at the point that it should be useful?
My daughter has a Junior Oxford Dictionary - she's 7. She is now able to use a dictionary correctly but pretty much every word that she looks up such as 'obeisance' is not in there (we're reading Children of the New Forest). Words she genuinely doesn't know or can't make a guess at from context are the ones I tell her to look up in the dictionary - and they're not there. In fact, the only words in there seem to be words a 7 year old would already know the meaning to.
Stealth boast aside - what's the word for something that becomes useless at the point where it is required? I know there is one and it's driving me mad. I have a feeling it may be something we've borrowed from German or similar.