Question: have the satellites ever gone wrong on the job?

Brian Weaver answered on 14 Mar 2019:

Yes, the positioning satellites have gone wrong on the job. The satellites send “health” information so we know if they are usable. One problem is when a bad satellite reports that it is healthy.
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Other problems can occur when an old satellite is replaced. For example, a few years ago an old GPS satellite was decommissioned and went offline. This caused a timing error for half of the remaining satellites and disrupted all GPS users until the problem was solved.
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In 2014, two of the European positioning satellites (#5 and #6) were launched into the wrong orbit.
http://www.esa.int/spaceinimages/Images/2018/11/Incorrect_orbits
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There are many other examples but those are some of the recent problems.