If your Sims are glitching out whilst using the water slide in the new Backyard stuff pack it may be because you have integrated Intel graphics. This is affecting both Macs and PCs but most of the reports are coming in from Macs running different versions of OS X. Intel chips can be found in 13″ MacBook Pros, MacBook Airs, MacBooks, entry level 15″ MacBook Pros, Mac minis and 21″ iMacs. So most Macs unfortunately.
The good news is that EA are aware of the issue and have managed to reproduce it. Let’s hope they fix it soon.
Update (22.07.16) – EA have just released a beta fix for this issue in the form of a mod to add to your game. Please read SimGuruModSquad’s post here for instructions and links – http://forums.thesims.com/en_US/discussion/895111/beta-fix-for-lawn-water-slide-animation-issue-on-backyard-stuff-pack
Update (26.07.16) – This issue has now been resolved in todays patch 126.96.36.1990.