There are ways:

• The encryption protocol might have a weakness
• One or both of the devices might be compromised
• The actual application design might have a weakness
• The actual application might be conpromised (i.e. on purpose rather than an unknown design flaw)
• The mechanism for generating the actual keys might have a weakeness (for example, for a while the symetrical key generation for HTTPS in the Mozilla browser was a lot less random than it was supposed to be so those connections were a lot easier to crack)
• The mechanism for distributing the keys might have a weakness

Ultimatelly the one trully safe encryption mechanism is the One Time Pad, and that requires a key as long as the message (hence why seldom used) distributed in a safe way (for starters, never over a public network) and there’s still the whole “compromised device” and “compromise application” risks (though implementing the One Time Pad protocol is stupidly simple)