Justin Bieber recently caught the attention of a few netizens during his appearance on "The Ellen Show" on Monday. It was not because he sang a perfect acoustic version of "Sorry." It was because he was wearing a Metallica t-shirt, which rock stars claimed to be a total insult to the band.

Metallica fans immediately took to social media to raise their concern and questions on why Justin wore a shirt with the band's name printed in front on TV. Some commented that the pop star wearing the shirt don't match at all. Meanwhile, others were not so kind and criticized the 21-year old.

The Wrap quoted some of the comments from rock fans. "Justin Bieber singing 'Sorry' on The Ellen Show while wearing a Metallica shirt is probably the least metal thing to have ever happened," one viewer said while another claimed, "Does Bieber think he's cute wearing a Metallica shirt on Ellen?"

A Twitter user named Andrew complained that Justin is a "fake" rock music enthusiast for wearing a Metallica shirt on "The Ellen Show." Meanwhile, others have different opinions on seeing the "Sorry" singer's outfit.

Another Twitter user named D.J.W thought Justin was pulling a "sick joke" on "The Ellen Show" for wearing a Metallica t-shirt.

Some days back prior to his appearance on "The Ellen Show," he was also photographed wearing a Nirvana shirt. The same complaints were hurled at the Canadian singer. NME.com quoted an individual who compared Justin's type of music with those from Nirvana and said, "Justin wearing a Nirvana shirt is an oxymoron because Nirvana actually [made] good music."

Regardless, it wasn't Justin's shirt on "The Ellen Show" that made headlines after. Instead, it was his interview with the show host Ellen DeGeneres about his ex-girlfriend Selena Gomez.

During his appearance on the show, the 21-year old revealed that three of his songs from his new album called "Purpose" is about Selena. He mentioned "Sorry," "What Do You Mean" and still an unheard song titled "Mark My Words" were about his relationship with his ex.

He also expressed hope to get back with the "Come and Get It" singer someday since they had good memories together. However, Justin also admitted that for now, they are both at the stage of doing some soul-searching so it remains to be seen when that romantic reunion will happen.