There are three ways to turn the ball over: turnover on downs, an interception or a fumble. But only the fumble carries a certain stigma with it, a lack of toughness attached to it. While turnovers on downs are essentially the result of the coach's decision, and an interception comes with the territory of being a quarterback (they're just trying to "make plays"), the fumble can yank a running back into the proverbial doghouse for months and years at a time. Compared to interceptions, the fumble is relatively rare, and since the blame is almost exclusively placed on the ball carrier, it tends to stay in the minds of coaches and fans alike. Ask Tiki Barber; before 2005, you couldn't tune in to a Giants broadcast without hearing the announcers wax poetic about his propensity to fumble.
Fantasy leagues obviously account for lost fumbles -- they are turnovers, after all -- but the bigger fantasy impact is not necessarily in the absolute point deductions for losing a fumble, but, for those backs fighting to be the feature back, the repercussions of the subsequent coaching decisions. Fumble problems often can be used as a means for confirming a preconceived notion of lack of toughness about a player. Tatum Bell is one of the best examples of this, as even though his stats -- a career 4.9 yard per carry average -- were impressive, and pundits expected him to break out, he has had fumbling problems throughout his career. Perception is reality, and the perception of fumbles -- "Look, he can't handle being a feature back, and the fumbles are proof!" -- is one all running backs must overcome. After all, a large reason why Barber's job was in question throughout his career was due to his fumbles; when he got them under control, the touches resulted as well.
With this in mind, I tallied how often a running back fumbled last season; how many carries, on average, did it take before he coughed one up? The threshold to make the list was 80 carries; the total number of carries a back had is listed as well. Total fumbles were used, and not fumbles lost; though obviously you would like your team to recover a fumble, there is little-to-no year-to-year correlation of recovering fumbles. Since it is attributed to luck, there is no need to penalize (or reward) a back who had bad (or good) luck.