January Jones' fans totally spoiled the penultimate episode of 'Mad Men' on Instagram

Matthew Weiner is one of the best in the business when it comes to obscuring spoilers for "Mad Men."

Try as he might, though, he can't control social media.

Common knowledge holds that Facebook and Twitter will be the most spoiler-heavy, and that hitting refresh on either of those timelines is foolish if you want the plot to remain a secret.

Instagram, however, is usually considered to be a safe space.

But last night, when the penultimate episode of "Mad Men" aired on AMC, spoilers came in the form of Instagram comments under a photo posted by one of the show's stars.

(Please don't read on if you don't want to encounter a spoiler yourself.)

January Jones posted a photo of herself in character as Betty Draper.

For anyone who hadn't seen the episode yet, the post in and of itself was not a spoiler, as it didn't have a caption.

But then Jones's followers filled in the blanks by writing comments like these:

instagram.com/januaryjones

It was clear to anyone who hadn't yet seen the episode: something really, really bad had happened to Betty Draper this week.

Fans were apparently not amused by this and other Instagram spoilers:

Tumblr users weren't safe, either, although that shouldn't be surprising due to the medium's propensity for drama:

Jones wouldn't be the first to spoil a plot point on Instagram — intentional or otherwise. "Game of Thrones" actress Lena Headey has come under fire for the same.