I had forgotten about this one, because I always blamed it on cancellation and other spells as well.

If you have compact set, and you hit enter/return, your prompt will display on the same line, as opposed to if you have compact off, it will not.

Example below.
Compact mode removed.

{02:40:39} <100%hp 100%m 100%mv 171tnl> <11 AM> (indoor-sleeping) -128hrs:
{02:40:39} <100%hp 100%m 100%mv 171tnl> <11 AM> (indoor-sleeping) -128hrs:
{02:40:40} <100%hp 100%m 100%mv 171tnl> <11 AM> (indoor-sleeping) -128hrs:


Compact mode set.
{02:40:59} <100%hp 100%m 100%mv 171tnl> <11 AM> (indoor-sleeping) -128hrs: {02:41:01} <100%hp 100%m 100%mv 171tnl> <11 AM>
(indoor-sleeping) -128hrs: {02:41:01} <100%hp 100%m 100%mv 171tnl> <11 AM> (indoor-sleeping) -128hrs: {02:41:02} <100%hp 100%m 100%mv
171tnl> <11 AM> (indoor-sleeping) -128hrs:


Pardon my rather long prompt otherwise. :)