Even top class players are sometimes guilty of it: reasoning from their own hand. The danger in doing so is that a player's defensive play is based on what he can see, but his partner can't. As a consequence 'the other side of the table' can go terribly wrong.
Take a look at how Brad Moss and partner Fred Gitelman allow an unmakable game to be made, when Moss doesn't realise the problem his partner faces.

1998, New York. The Cavendish is a big money event. Every deal's score is compared with the scores of all other tables in IMPs. Meaning there are a lot of IMPs to be won or lost at each deal.

S/All		♠	8 5
		♥	8 7 6 3
		♦	A 7 6 3
		♣	J 5 4
♠	Q 10 9 7			♠	A K 2
♥	J 2			♥	10 4
♦	K 9 4			♦	Q 10 8 5 2
♣	K 10 9 7			♣	Q 6 2
		♠	J 6 4 3
		♥	A K Q 9 5
		♦	J
		♣	A 8 3

Quantin's 2♠ bid showed a second suit and was a game trial. Having an extra trump (the French open five-card majors, so North could have raised on only three hearts as well) and a useful doubleton in spades (so he hoped), Abécassis thought he had enough to bid 4♥. Critics will call it either a brave or reckless bid, depending on the result...

Moss led the ♣10. Declarer won with the ♣A, cashed a high trump and continued with a small spade from hand. It looked like EW could (and should) simply take two tricks in each black suit. How could it go wrong?

Moss won the third trick with the ♠9. The first trick had made it clear that his partner had the ♣Q. He continued with the ♣7. A big mistake. He knew the club layout, but his partner didn't. Gitelman thought his partner had ♣10987 in clubs (though one might ask why West didn't play back the ♣9 or the ♣8 from that holding) and declarer ♣AK3. So he let West's ♣7 ride by ducking!
Declarer was surprised to make the ♣8. He lost one more spade and was able to ruff two spades in dummy: ten tricks.
NS won 297 IMPs on the deal.

This disaster should have been avoided. West doesn't even have to cash the ♣K in order to continue with a small club (that would be embarrassing if East had started with only one trump and ♣Qx — though this layout is impossible in view of the bidding). All he has to do is play back the ♣9 instead of the ♣7. Nothing can go wrong. Partner will let the ♣9 ride if dummy ducks. And if dummy covers with the ♣J, East of course covers with the ♣Q.
The lesson is clear: make life easy for partner!