What is considered Mechanical Failure

I had the RNY done in 1999 and never hit my goal weight.  I see all this talk about Mechanical failure.  Can someone tell me how you know if you had Mechanical failure?

Thanks

Mine was documented as esophageal erosion due to the inclusion of a prolene mesh band at the top of the stoma, necessitating removal to prevent Barrett's esophagus.

I've heard people say it was covered for staple line disruptions.