What's a "red flag" in a TV show that tells you it's about to go downhill?

Though there it really was a core component, so obviously itllt work

unless the show is specifically about time travel.

Even then... I mean, what has good chances to be OK is when the show is a pure serial with a defined story with a start and an end (so basically a looong movie cut in 40 minutes pieces), in a universe where the conditions of the time travel are clearly defined and limited.

When it is a series-serial mix (like >90% of TV shows theses days), with extra seasons which may or may not be ordered, you can be sure the writers will trip up, as they have to invent new things, and some of those things will break the conditions and the limits defined earlier and then it won't make sense anymore.

Fucking La Brea