Hello everyone!
I have been using BobCad v26 with their Mach3Metric post processor for like 8 years without troubles. A few days however, something very strange started happening. Attached is a simple hole pattern. When I run it it makes a row of holes that aren't in the file. I also made a few other files where the tool failed to retract when it should. All .tap files I wrote before two days ago run as they should, and I haven't made any changes to the Mach3 configuration or settings or anything. Any ideas?

I count 20 holes from your .tap file - how many are you getting ?

Tweakie.

Yes 20. It drilled those properly, and then went on to cut 7 more at around X70. Y positions similar to the X78 holes but off by around 10mm. 

I don't know if this will help but assuming you have something corrupt within Mach3 try Operator / Restore Settings and select a time before the problem surfaced. You can also restore to an earlier date by manually selecting (and renaming) an .xml file from your Mach3 xmlbackups folder and transferring it to your Mach3 folder.

Tweakie.

I am sure you are aware but before doing anything it would be wise to make a secure backup of your Mach3 folder before making any changes to settings, etc. - then there is always a way back if everything goes pear shaped 

Tweakie.