Sorry, I should have been more precise. It doesn't go off all the time, but it does hit one of the basic arithmetic tests at the start of the manual. I dug into this a bit:
```
In[1]:= a b c          (* works *)
In[2]:= (a b) c        (* works *)
In[3]:= a (b c)        (* asserts *)
```
If I understand correctly, it's asserting that a nested `Times` can't appear on the RHS of a `Times` expression on input. That's true most of the time because `Times` is left-associative, but it might not be true when the input contains parentheses.

IMO this assert should just be removed. In any case, it goes off on master if it's fixed there, so I don't think it's related to this PR.