[Cheshire Cat]

Ranger guards (guards with crossbows) cannot be pick pocketed. For example, the one near the west Falador bank.

Also, after engaging in combat if you attempt to thieve an NPC (before attempting to leave your current tile) you will be stuck in place.

Example; You attack a Market Guard, defeat it and click on a Master Farmer to pickpocket. You are unable to pickpocket the Master Farmer and are stuck in place until moving your character to another tile.

[Gornu]

I'd honestly think that attacking anything in an online period and starting to train thieving by pickpocket causes this, the engine confuses that pickpocket action to combat, specifically mimicking ranging or magic and prevents you to move to steal from the npc.  This affliction goes until you log out.  Even if you use the empty quiver and bow trick to prevent you from attacking an npc you pickpocket from, the only way out of this scenario is to relog.

[Flustered]

This has been forwarded to the developers.

Reference: blitz/thieving#13

- Flustered