AT&T accidentally streamed the highly anticipated Season 8 opener of "Game of Thrones" four hours early for some customers on Sunday evening.

Most people needed to wait until 9 p.m. local time to watch the episode. However, news website Insider spotted people on Twitter who were able to watch it beginning right around 5 p.m. ET. While it's a relatively minor flop, it's an indication again that streaming services are still struggling with reliability.

AT&T owns HBO through its acquisition of Time Warner, which was completed last year after a year-long merger process. The Department of Justice attempted to stop the merger after it went through, but its case fell apart in February.

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"Apparently our system was as excited as we were for 'Game of Thrones' last night and gave a few DIRECTV Now customers early access to the episode by mistake," AT&T told CNBC. "When we became aware of the error, we immediately fixed it."