Let's Get It Out Loud
Ed Sheeran is facing a lawsuit for his smash hit Thinking Out Loud, allegedly a copy of Marvin Gaye's classic Let's Get It On. But in contemporary music, almost everything seems to have been invented already. If you grew up listening to a certain band, there will probably be an inexorable flavour to your music that some may call plagiarism. Even the most creative may try to emulate the sound that inspired them to do music in the first place. Other times their subconscious may play a trick on them by ripping off a tune. The issue lies in where and how to you set the limit between inspiration and copying.

Eight same consecutive notes, substantially similar rhythm and identical chord progressions all constitute the copyright features that will help determine whether a song should be considered or not a rip off. However, it turns out that hundreds or even thousands of songs share the same chord progressions, typically characteristic of a certain music genre. During the 1950's and 1960's the chord progression C-Am-F-G in the key of C became so popular that it helped define a sound. Stand By Me, Up On The Roof, Blue Moon, Unchained Melody or Those Magic Changes all could have been considered too similar.
Likewise, Australian comedy band Axis Of Awesome released a video portraying some of pop music's hottest hits sharing chord progressions, including Torn by Natalie Imbruglia (originally by Ednaswap), I'm Yours by Jason Mraz, With Or Without You by U2, If I Were A Boy by Beyoncé, She Will Be Loved by Maroon 5 and many others. That is why the law contemplates that identical chord progressions don't always make for copyright infringement.
But even in Sheeran's case, his song doesn't completely share Let's Get It On's exact chord progression. Additionally, both songs' melodies are completely different. Avicii's Wake Me Up's verse's melody is way more similar to that of Emeli Sandé's Next To Me and it hasn't awakened any plagiarism suspicion. Sam Smith's Stay With Me on the other hand was accused of copying I Won't Back Down by Tom Petty for just a few notes in the chorus.
Other songs, like Lady Gaga's Born This Way, considerably similar to Madonna's Express Yourself get luckier, as Madonna herself decided to cover the tune in one of her tours instead of filing a lawsuit.
I get that Blurred Lines was probably a Got To Give It Up rip off, but maybe this time things have come too far. I'm far more of a Marvin Gaye fan than an Ed Sheeran fan, but I can't help to find the copyright infringement allegations somehow unfair. After all, Sheeran has managed to write a pretty good melody of his own. The bass line and drum section nevertheless are clearly reminiscent of Gaye's classic. Of course, there's always sampling, but that's a whole different story.