Thank you guys answering my question!
I guess I get some sense from your answer.
Yes, I mean "unwanted effect". Actually I think of case, like if I send pulse train "1000111....", and I have source output imp. 50Ohm, TX line Z0 100ohm, load 200ohm. then for high f, there would some reflection due to the mismatch, the bit signals at later train would be affected by the previous bit signal reflection. If the TX is short compared with transmitted signal wavelength (low f), the reflection would not play effect because the phase will not change, is that what you mean?
many thanks!