Should the Bluebirds go down at Sheff Wednesday today then they would still make the play offs if either Burnley or Preston fail to win.
If Burnley only draw then Cardiff would need to lose by 5 goals to allow the Clarets to ovetake them. A draw for Preston will see them miss out.
Wins for Preston and Burnley and defeat for Cardiff will see the City miss out. Unless there is a combination of City losing by a single goal and Preston winning by a single goal but with Cardiff scoring more (i.e. Preston win 1 - 0, Cardiff lose 2 - 1 - then City would be in the play offs with a better goals scored ratio than preston).
You only have to look at Northampton yesterday to see how easy it is for a team to blow it. The Cobblers were above the relegation zone at kick off needing a point to stay up, but they lost at Leeds and the two teams below them both won to condemn Northampton to League 2 next season.
Now we are in no danger of relegation of course but we are in a position where we can blow a play off place if we don't get a result.