The Berenstain Bears is a children's literature franchise created by Stan and Jan Berenstain and continued by their son, Mike Berenstain, who assumed partial authorship in 2002, and full authorship in 2012 following Jan's death. The books feature a family of anthropomorphic grizzly bears who generally learn a moral or safety-related lesson in the course of each story.
This may be the most widespread Mandela Effect . The controversy is centered on the spelling of the word, particularly the "-stain" suffix, with many remembering it being spelled "-stein"; i.e. "Berenstain" compared to the widely remembered "Berenstein"
ResidueThere are many photographs online showing the spelling as "Berenstain", although a number of them have been proven as fakes.  
Mandela Effect Database
The most common explanation for this particular Mandela Effect states that because the "-stein" suffix is much more prevalent than the "-stain" ending, many people simply confuse the two and assume that the name is spelled with the much more common "-stein" suffix. This idea has been tested by Reddit user KitKhat, where he performed a (relatively) blind study where he showed people who have never seen the show the Berenstain logo, and asked them to take note of it's spelling. When asked later, they confidently state that it was spelled "Berenstein".  This hypothesis is further proven by various websites constantly referring to the literature as "Berenstein", even though the website itself contains many photographs showing the proper "Berenstain" spelling. 
- Berenstein Bear dolls with both spellings | https://www.reddit.com/r/MandelaEffect/comments/70klzv/berenstein_bear_dolls_with_both_spellings/
- Holy shit - found Berenstein evidence while packing | https://www.reddit.com/r/MandelaEffect/comments/4w83y2/holy_shit_found_berenstein_evidence_while_packing/
- Did a Berenst*in test | https://www.reddit.com/r/MandelaEffect/comments/3gh0qa/did_a_berenstin_test/
- Seanbaby.com | http://www.seanbaby.com/nes/nes/egm20.htm