What happens when I connect my oscilloscope to a circuit

I have a circuit that uses a 16F877 and I was having trouble getting my chip to send data out serially on a couple pins. It would work on c.6, but when I tried B0 or B2 it basically did nothing. While trying to figure out what was going on I connected my Oscilloscope. As soon as I connected the ground line the circuit started sending data out as I intended. the probe end wasn't touching any part of the circuit, just floating in air. When I disconnect the ground, the circuit again doesn't work right. I have tried adding a couple different size caps, with some varied success, but nothing definitive. My circuit is powered by a 9 volt brick, so there isn't a true ground on the circuit, but I've never noticed problems before with that setup. Anyone have any ideas?

Couple of things to check on. Serial out + ground are connected to the device you are sending to. You are possibly missing the ground. Second to check is decoupling caps on the circuit.

High likely one or 2 missing Vdd, Vss connections or a floating MCLR.

Could still be some weirdness from your crystal...

You were both right, the ground line was missing from the terminal to the PC. It was on the schematic, but altium didn't update the board correctly for some reason. Thanks for the responses.