I have an odd problem. I use Tik to create a pptp connection between our office and some home users. The cable internet people work fine, however the DSL people cannot pass traffic over the pptp connection. These DSL people have a DSL modem which does the PPPoE authentication on the modem itself and then gives a public IP to whatever is plugged into it. (Our cable internet does not use PPPoE authentication). Both the cable and the DSL are dynamic IP setups.

If I bring the PPPoE connection back to the Tik and set that as the default route, my PPTP connection works perfectly. Does anyone have any idea why having the PPTP connection one hop behind the PPPoE connection would break it? Also, is there another tunnelling protocol where this would not be an issue? I am not exactly attached to the PPTP tunnel, I just arbitrarily chose it, I simply need a VPN of some kind.

Sounds very much like http://forum.mikrotik.com/viewtopic.php?t=9825

Make sure that your router facing the internet is re-writing MSS to the lowest in the chain and not actually making it bigger AND that it is actually getting out to the internet.
In our case the magic MSS number is 1400 for users behind a PPTP tunnel in turn behind and internet connection.

On your router you have two interfaces, set proxy-arp on internal iface.

And works ! 1460 for PPTP MTU/MRU is OK if you have PPPoE with 1492 MTU (in some cases i see some ISP`s set their low then 1492 but workin` )