Advanced Divvery occurs when you really think you know what you're doing, and what's more you really should. Observe and learn.
Fixing up MiL house pre-sale we discover we were suffering from an excess of SiL's predilection: "Chuck it out, it's a load of rubbish" which in one particular case included the keys for the patio doors.
As it would have been mean not to, and replacing a euro lock is a cheap thing, I decide to carry out this operation. Even drilling the lock out, it should only take about 30 minutes. Using FiL's cheap drill that shattered in the lock added a few minutes. But, once the lock was unlocked, how long do you suppose it was before I remembered that there's a single locating screw in the middle of the cylinder, without removing which you will get nowhere? Yes that's right. Quite A While.