This may be obvious to experienced shortwave radio users, but I'm posting it becase I'm new to the hobby and I just learned a lesson. If you use a reel antenna, always reel it in slowly. One revolution per second would be a good idea. Also try to keep the wire somewhat taut during reeling. Following these tips can avoid nasty tangling situations.
A reel antenna that I own had its wire caught under two loops, and this was most likely caused by hasty reeling. I could only get several inches of the antenna out of the canister before the snag prevented further unwinding. I turned the reel back and forth, and tried tugging and loosening the wire, but it was hopelessly stuck.
I ended up removing the screw at the center of the reel to disassemble the casing. This allowed me to loosen the correct loops to get the wire free. Then I unwound the entire antenna, gently worked the kinks out of the wire, and rewound it slowly and with some tension. Problem solved, but time wasted.