I stopped using this option a long time ago because in my experience it rarely works… 
Why?
Attached a very simple example I tried this afternoon…
–Mitch
ExBoundFail.3dm (69.3 KB)
fwiw, that works for me (sort of)… i did find something a little weird though–
if i select the curve (either before or after running the command) and i don’t move it with the cursor prior to choosing ‘To Boundary’, i’ll get an error- “One input was not extruded because it would have made an invalid surface.”
if i move the curve a little (previewing the extrusion) with the cursor prior to choosing To Boundary, it will work as expected.
??
are you able to duplicate this behavior as well Mitch?
Nope… The command line just says the following:
Select curves to Extrude. Press Enter when done: Extrusion distance <0> ( Direction BothSides=No Solid=No DeleteInput=No ToBoundary SplitAtTangents=No SetBasePoint ): ToBoundary Select a boundary surface ( Direction ): Splitting... Press Esc to cancel
–Mitch
OK, now I see something - you are right, it is the on-screen direction of the extrusion that is causing the failure (good catch!). As I have the command line on the bottom, if I go to click the option on the command line, it fails (as it is pulling the extrusion downwards), if I pull it upward and just type the option, it works. And yes, if I don’t pull it at all I get the error message you indicated.
So, this is totally buggy - it should absolutely NOT depend on what’s on the screen…
–Mitch
Hmmm- I cannot make it fail so far.
-Pascal
try not moving the preview surface…
on mac, do exactly the following:
• type ExtrudeCrv (return)
• select the curve (return)
• type ‘t’ (return) [‘T’ being the To Boundary option]
• click on the surface
this should give you the error i reported earlier.
likewise, if i do use the mouse to preview the extrusion and i preview it downward (whereas the boundary surface is in the positive direction) then use the ‘to boundary’ option, the extrusion will fail… i have to have a preview of the extrusion going in the direction of the boundary surface in order for the command to work properly
Weird. Just did the exact same thing as in the video and it is working here. Tested it with typing the command and in the menu. With typing T and clicking it. 
I can’t reproduce this 
Is your command line on the top or bottom?
–Mitch
on the Top. Maybe that can be the cause… ill check it…
Oke on the bottom it bugs as well xD
and the thing is… when I moved it from bottom to top again and use T to use the boundary thing. It didnt work until I clicked the command in the command bar again. Next time when typed T. It worked again.
Sorry. So bad English there xD but hope you can understand what I’m saying