As an FTE I have designed quite a number of fixes, and in one recent case quite a substantial portion of an aeroplane. It doesn't present too big a problem IF you ensure that somebody else is there to oversee what you're doing. The problem is, particularly as a certification FTE, if you designed the fix you can get too close to the solution and subconciously refuse to see it's own faults.
So it really comes down to good practice. If, as the certification FTE who also happens to be the design signatory (which I often am) I were to design and approve something without somebody else having sight and veto over what I've done then frankly I deserve to be shot. Sometimes as JF says I do find myself in the best place to design the fix and have been seen sat in the middle of an aircraft factory with a drawing board. But, it is absolutely vital that I then ensure somebody competent to do so has overseen my own solution.
G