Shabbos 61b - Eid Echad Bi-Issur Dirabanan

 לרפואת האדמו"ר ר' יצחק מנחם בן גיטל מירל בריינדל לאה
בתוך שאר חולי ישראל
Rashi says that if a person wrote three amulets which were successful he is now considered an expert and all future amulets may be carried on Shabbos.

The Rosh was less that thrilled with this understanding. Just because he wrote three good amulets does that make him an expert in all amulets??

The Pnei Yehoshua defends Rashi and says that since he is מוחזק that he healed three times we should believe him that he is an expert based on the rule עד אחד נאמן באיסורין. Moreover, here there is only an איסור דרבנן involved as the Mishna says on דף ס. The P"Y goes on to say that here there is a chezkas isssur of Shabbos and maybe עד אחד is not believed against a חזקת איסור דרבנן and that would be the rationale of the Rosh.

Many Achronim however, say that עד אחד IS believed even if there is a חזקת איסור.

If you would like more information on this topic see the sefer מאור ישראל חלק א עמוד קיד.

[A question the Achronim ask - Even if someone isn't an expert, why is it forbidden for an ill person to carry his amulet on Shabbos, there is at least a ספק that it will be effective and ספק נפשות להקל?]