Is it only the one time you had it 42 days? Cause if its only once then you should be ok, as the last post mentioned.
I will say though, that the first cycle it only registers everything as its using that data to get to know you. And btw, i stopped temping as well for the same reason.
I have had really good results with it so i can only go by my info. I will say this though, after my mc, as the cbfm had not been in use obviously, it had to learn things again..but is now reading me very well. ( i used to have longer cycles but after taking agnus castus, b vit complex and primrose they have shortend and the cbfm is now asking for sticks earlier than before)
I dont know about it being wrong at this stage..but when in doubt

is our motto here at b&b, lol!!!..if you have ewcm..go for it mrs, as cp can be a bit unreliable at times..dont get me wrong, but it does have a mind of its own, whereas ewcm is controllled by estrogen which your egg releasing follicle produce at time of (and after sometimes) ov.
I also credit it for getting me pg as my predicted ov date was sun and i got a peak on thursday so was busy that week- only twice, but it worked!!!
I would tell you to have a look at this but the site is down at the mo'
http://www.clearblue.info/uk/FAQSFAM.cfm
Good luck and i hope ive been of some use if any...

Omi xxx