I have created a part and threaded each end according to 6 5/8" API Regular Left Hand thread specifications. I attempted adding a fillet to both crest edges of the threads on one end of the part with one fillet feature and received a rebuild error. I then applied the fillet to only one edge and received no error, so I created a second fillet for the other edge and still no error. Strangely, when adding the fillet to the threads on the opposite end of the part with the same geometry I am able to re-use the first fillet feature for the corresponding edge with no error. However, if I use the second fillet for its corresponding edge at the other end, I recieve a rebuild error. I even tried making a third fillet for the remaining edge and still get the error. It seems that this error is only present intermittently as I have created similar threads on other parts with no errors at all using only one fillet, and somtimes I will get this error when I first try to fillet and on later attempts I won't. Thanks in advance for any help.