A new one for us!
When using ® © or ™ (both numerical and named entities) in ALT tags (title tags were fine), Office 365 and Office.com do not display the linked image, but instead displays an R, C or TM image in its place. We also tried pasting a (C) glyph. Same error.

EOA tests were inconsistent as to which version (IE 10, 11, Mac, FF, Chrome, etc.) this error appeared in, but we were indeed able to reproduce this error in live testing on a PC version of 365.

We tested other entities (é etc.) and those all test fine.

Thank you Microsoft!