First a few questions..

1) What year, model and engine are we talking about?
2) Does the 'mechanic' work at a dealership? Or just a local garage?

Now some information.
There is a known problem with the front crank bolt on the 1989 thru
1996? 6G72 SOHC engines. Mitsubishi redesigned the front crank bolt
and issued a technical service bulletin for the 1992 thru 1996? 6G72
SOHC engines. Mitsubishi did not include the 1989 thru 1991 6G72 SOHC
engines. But, the 1989-1991 engines have seen the same problem.

The problem is that the front crank bolt begins to loosen. This allows
the pulley to become loose, then wobble. A lot of times the crank is
damaged. Some times the bolt breaks off in the crank. If it is caught
early, everything can be repaired. Once the bolt breaks or the front
of the crank is chewed up, the repair price goes up a lot.

To answer your question about the 'sloppy job' causing the problem. If
you consider it sloppy that someone did not know enough about the
engine to replace the bolt. When it's fairly common knowledge that the
6G72 engine has this problem. Even Mitsubishi issued a TSB that covers
most of the model years.

A good mechanic or a Mitsubishi dealership should replace the old
designed bolt with a new bolt whenever the timing belt is done. The
new bolt is in the $5 - $7 range. It's cheap insurance.

Go over to www.4x4wire.com There is a great online forum there for
Mitsubishi truck owners. Do some searches on 'crank bolt'. You will
find everything (and more) that you wanted to know.
Everything after the @ is spelled backwards.