As someone who works in a uni, I can't see how this could happen. to me it sounds like empty threats from a scared new lecturer.
All exams / assessed work are written and marked anyonmously, so it would be impossible to identify 'troublesome' students over any one else who took the course. If the lecturer was sacked, then there was obviously strong, proveable evidence that they were being rascist, otherwise they uni couldn't get rid of them.
all exam papers etc are also marked by a combination of internal and external assessors (the externals will never have come into contact with the students) so it should be a fair assessment process.
If any of the students have reason to doubt their results, then there is usually an appeals process in place to deal with complaints. although i very much doubt it should come to this as universities are v. careful not to get tangled in any lawsuits.