The saying is 'You've got another think
coming.' Many people, however, write incorrectly 'You've got another thing
coming.' This kind of misunderstanding is called an eggcorn (which you can look up in Wikipedia or on the Eggcorn Database).
The meaning is as Robert described in #3 (and in the link I gave in #4). It means 'You are going to have to rethink that / think about that a second time, because you're simply wrong.' For a translation I would suggest something likeA: Aber ich dachte, ...
B: Na, (wenn du das denkst,) da irrst du dich gewaltig.
It might be useful if LEO could add a second translation more like this for 'You've got another think coming,' if German speakers agree.
If I understand correctly, 'Du bist auf dem Holzweg' is somewhat different because it's more neutral, without the overtone of impatience or annoyance. It might sometimes overlap, but it's probably closer to some of the other translations in LEO, e.g.,You're barking up the wrong tree
You're on the wrong trackDictionary: holzweg
'Woodway' is an uncommon word in English. If anyone told you there was an English saying with 'on the woodway,' they were probably teasing you. They (or you) might enjoy this thread:related discussion: disgusting 6-gear menu - sechsgängiges Degus...