Perfect crosstalk?

Weekend fun:

(I know it's not best practice, don't tell me... :) )
Setup: only one cat5 (solid core, around 50-60m run) copper to a classroom, need access for lecturer PC and wireless AP. Instead of small managed switch, I went with a passive splitter.
AP receives 24V POE on far end after splitter.
AP immediately falls back to and stays on 10meg connection, error counter on 0 after days, appears to be working perfectly, wireless clients happy.
Fun part: sometimes the MAC of the AP appears on the 'wifi vlan' port (good thing), and also on the 'student vlan' port where the split cable PC is.
Double-checked, vlans are not bridged together in the area.

So: what's your guess? Perfect, full-frame crosstalk on the splitted cable, or the otherwise perfectly reliable 3Com 4400 box started bleeding between vlans, or other?

Cheers,
Tamas