In the case of Andy Coulson, David Cameron got it badly wrong as he himself was force to admit this afternoon.
It was suggested back in July 2009 when the then Leader of the Opposition hired Coulson as his head spin doctor - despite the fact he had resigned from editor of the News of the World for breaching people's privacy - that he was making a big mistake. The suspicion was that it was in some way a favour to keep Rupert Murdoch, the media mogul, sweet.
During his time at No 10, Mr Cameron was keen, as indeed he was again today, to point out how no one had a single complaint against Coulson in the work he did at the heart of power. Indeed, the PM was also keen to point out after the 2010 election how this had not been the case with Labour press chiefs Alastair Campbell and Damian McBride. He obviously cannot take the high moral ground now.
So has the PM been damaged by this? Yes. Fatally? No.
But Labour will no doubt seek to make as much political capital as possible over it, particularly as Ed Miliband made great play in standing up to the News International empire during the Leveson inquiry. The Labour leader this lunch-time insisted an apology was not good enough; Mr Cameron must explain why he famously gave Coulson a second chance.
For the public, the fall-out to the hacking scandal might make many voters reconfirmed in their low opinion of the Conservative leader, who last time round failed to get a majority despite the tired-looking premiership of Gordon Brown, which was in charge when the financial storm hit.
The latest controversy might just add to the cumulation of events that tend to hang onto a Prime Minister as he or she trudges through their time in power. It could be another factor when people cast their vote next May.
As for the present, Mr Cameron has to face Mr Miliband at Prime Minister's Questions tomorrow. Hold onto your hats, it could be a bumpy ride.