Was playing a mini-game on a server. Before start of game, all players had (among other effects) slowness128, jump_boost150, and invisibility. Then as game starts, it runs 'effect @a clear' to clear all status effects. Worked for everyone except this one person, whose client shows the potion effects disappear from the HUD and inventory screen, but was still slow, unable to jump, and invisible. (Even invisible to other players!) Logging out and back in fixed it. Video evidence here:

https://www.youtube.com/watch?v=K6kkidVzdcw&feature=youtu.be&t=1m44s

There's a timestamp in the URL for 1m44s, that is a few seconds before chat says 'Start! Go!' and you can clearly see the HUD change in the top right of the screen that potions disappeared, but also clearly see that the player is still slowed and invisible.

I originally thought this was a client-side glitch, but the fact that the player is invisible to other players (see at 3m30s) suggests server-side involvement (or a flawed network protocol).

But the fact that a re-log fixed it (after about 4m35s in the video) suggests a client bug.. who knows!