Scrolling through weapons while running too many times, causes them to became unuseful. After stop, you cannot shot, even though the gun is loaded. This is not a cooldown time caused by weapons switch, as it is permanent. Only fix is to die or repurchase character (not refill).
Trigger: Take Rocket soldier, Sprint and keep scrolling up and down for a while. After pressing LMB, you cannot shot at all from both weapons.
UPDATE: Apparently, it is caused by weapon switch cooldown. While sprinting, switching weapons causes cooldown time to add up instead of refreshing it. While stationary or jogging, cooldown works perfectly fine. Inability to shot has duration and it depends on amount of scrolling I do while running. After about a minute, I can use weapons again.