When you walk along the street and see a ladder up ahead what do you do? Do black cats walking across your path get a reaction, or maybe seeing a lone magpie?
If you pay attention to old wives' tales, then maybe these situations make you stop and think every now and again. I would be lying if I said I was any different.
The number 13, of course, has that same sort of ominous portent. Certainly this was the case at the RDS less than a month ago when Ulster were defeated by Leinster.
In that match, the 13th minute signalled the worst possible start that Ulster have made all season, as Shane Horgan crossed for Leinster's third try.
A 17-3 deficit at that stage against most teams is recoverable, against Leinster it is a chasm.
While Ulster made a decent fist of working their way back into the game and won the remaining 67 minutes on the scoreboard, it was in those opening stages that Leinster shredded Ulster's defensive line to pieces.
Usually, in a competition, it is difficult for a coach to ensure that his players do not look too far ahead, if at all.
Ulster coach, Brian McLaughlin, will have no such problems this week, as his players will not be focused on anything other than the 7.05pm kickoff on Friday night. It is important that they show that they have learned lessons from that last outing and, above all, the first 13 minutes have to look substantially different.
It stands to reason that Ulster cannot let Leinster off to the same flyer. If the team can achieve this, they have a chance.
Ulster's early defence that night was porous, not because of missed tackles per se, but rather due to a lack of composure, patience and intelligence.
The tempo with which Leinster hit the players stunned some into mistakes that they would ordinarily not make.
There is no reason for them to panic - we have witnessed plenty of occasions this season to prove that when Ulster's defence is aggressive, patient and well-organised, the defensive line is hard to break down.
I am going to state another obvious. The breakdown will be as keenly contested as any this season. Leinster are a team fuelled by high octane and thrive off quick ball. Slow that ball down and their ability to create or take advantage of holes is limited.
With Leinster unbeaten at home this season both in the Magners League and Heineken Cup this will be an enormous battle for the Ulster players.
Yet, Ulster need to embrace this mood as, if they are to create the circumstances in which they can win the game, they have to generate a battle-type scenario.
The reason for this is one key fact going into the game - Leinster have a Heineken Cup final eight days later.
It is to his eternal credit that Joe Schmidt has not approached any of this season's big games with anything other than the utmost seriousness, and it will not be any different this Friday evening.
However, let's not kid ourselves, in terms of standing, individual and team goals, a Magners League victory does not compare to a Heineken Cup trophy.
Ulster need to get the game to a point, where they put enough doubt in some of the Leinster players' minds that they start turning their thoughts to the Millennium stadium and Northampton Saints. It would be the most normal human reaction, but it will take Ulster pro-actively to trigger such a response
Everything now is a bonus for Ulster and the players have absolutely nothing to lose, but their normally unflappable opponents do, and this might just make a difference.
Ulster need to arrive with with a potent mixture of fire and ice.
No doubt, Friday night signifies 'red alert', or maybe it should be blue. Whatever colour you prefer, Leinster, on current form, provide what can only be described as the ultimate test.
Nevertheless, if any of the Ulster players are superstitious, they will take heart from the fact that the fixture is being played on Friday the 13th. Cross your fingers, get a horseshoe, find a four leaf clover and even let a bird poo on your shoulder, as Friday night is as tough as it gets.
Some team, sometime will take away this home record, so why not our boys this Friday, and in the process qualify for a Grand Final.