There's worse. When your trait adds function foo and your code does obj.foo(), if the underlying type later adds a foo method, compilation breaks. This is very common with traits that usefully add useful methods that are missing in libstd... which break when said method is finally added there.
I ran into this case yesterday, for some cases there are warnings now. In this case I used ".div_ceil(...)" from from the Num [0] crate.
(some_integer).div_ceil(&2)
^^^^^^^^
= warning: once this associated item is added to the standard library, the ambiguity may cause an error or change in behavior!
= note: for more information, see issue #48919 <https://github.com/rust-lang/rust/issues/48919>
= help: call with fully qualified syntax `num::Integer::div_ceil(...)` to keep using the current method
= note: `#[warn(unstable_name_collisions)]` on by default
