You have asked tosfos kashe. It is rather complicated so let me try to explain what exactly he forgot. If he forgot he had a needle but knew it was shabbos then it would be misasek. But tosfos answers he would remember he had the needle but would forget it was shabbos when it would be a 'shogeg'.
The meiri says that even if he forgot he had the needle and remembered it was shabbos when it is misasek in this case you would make a gezaira since if he would remember the needle it would be chayev misa and rovos case is different.
The pnai yeshuo explains this because rovo's case is a karmelis where one can never be chayav misa whereas our case is a 'street' where one can be.
The sfas emmes says that the example of the stone is different because no wants to carry a stone. Whereas here the tailor wants to carry the needle. This is a chiddush in the ptor of mleches machsheves. That it only applies when 'loi nicha lai'.
This is all taken from the mesivta gemoro where my present posts on daf yomi psochim, are all taken from.
Why one may go with t'fillin and we are not scared he will also forget it is shabbos and carry them, is a separate discussion.