Football isn't a difficult concept; if the ball crosses that big white straight thing then it's a goal for whichever team did it. But over the years some referees have become confused.
On Sunday Juan Mata was awarded a goal for Chelsea in their FA Cup semi-final win over Tottenham despite the ball seemingly striking John Terry, who was sprawled out on the line, and not going in.
But it's not the first time (this season even) that there have been controversial calls. Here are some of the best.
1. Mata v Tottenham
So here's Juan Mata's 'goal' against Spurs in the FA Cup semi-final. It put Chelsea 2-0 up and although Tottenham pulled a goal back, the Blues went on to win 5-1. Did it cross the line? Referee Martin Atkinson thought so.
2. Clint Hill v Bolton
Martin Atkinson involved again here as he denies QPR's Clint Hill a goal at Bolton despite it clearly crossing the line. Oops.
3. John Eustace own goal v Reading
Perhaps the most bizarre ghost goal ever - the ball bounced off Eustace and went behind for a corner a, good four feet wide. Referee Stewart Atwell gave a goal. Cue confused faces everywhere.
4. Pedro Mendes v Manchester United
This one is memorable on two counts: 1) the distance of Mendes' shot and 2) Roy Carroll's 'what? me?' face after fumbling the ball a good 400-yards over the line.
5. Frank Lampard v Germany
We all know this one, right? Lampard's dipping volley bounced well over the line in the 2010 World Cup knockout clash with Germany. It would have made the score 2-2 and England went on to lose 4-1.
6. Clive Allen v Coventry
Before continental square nets we had these things can stanchions, and the very word must still make former Crystal Palace stiker Clive Allen shudder, after this 25-yard effort against Coventry City was ruled out.
7. Freddie Sears v Bristol City
Poor old Palace don't have much luck do they? In 2008 Freddie Sears opened his account for the Eagles but it hit the back of the net and bounced out. The ref said no, and Freddie never scored for Palace again.
8. Paddy Connolly v Partick
More stanchion fun here as Connolly scores for Dundee United only for referee Lee Mottram to rule it out and wave play on, despite the fact a Partick defender catches the ball. Facepalm.
9. Sulley Muntari v Juventus
A clear goal this one for the former Portsmouth man, but it wasn't given. It came just days after Hill's disallowed goal for QPR and the internet literally exploded.
10. Jonathan Howard v Middlesbrough
Perhaps the saddest disallowed goal ever here as the Chesterfield striker had a perfectly good goal ruled out in the classic 1997 FA Cup semi-final. The Spireites lost the replay after a thrilling 3-3 draw.