Once got a 1 1/4" BSP bottoming tap jammed in a 316 stainless coupler that was painstakingly welded onto a large chemical reactor vessel by a very moody fitter.
Completely forgot to use some trefolex. The thread edges on the taps flutes were blunt and the couplers threads were heat warped so they pricked up, wedged the tap in situ half way through and I had to grind the bastard lot off and deliver some bad news to the fitter.
(Finster Ate a cat biscuit for a bet, Fri 22 Mar 2013, 20:53, Reply)